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Unable to use classes of one child module into another in multi-module maven project

I want to use the classes of one child module into another child module present in multi-module maven project. I have followed many existing solution. I am doing exactly same thing as in existing solution. But still it does not works.
Project structure:
Parent A: sampleproject
|-- child A: Gui
|-- child B: calculator
This is how pom.xml of parentA looks like:
<groupId>com.example</groupId>
<artifactId>sampleproject</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>pom</packaging>
<modules>
<module>Gui</module>
<module>calculator</module>
</modules>
pom.xml of child A looks like:
<parent>
<groupId>com.example</groupId>
<artifactId>sampleproject</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>Gui</artifactId>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>${project.groupId}</groupId>
<artifactId>calculator</artifactId>
<version>${project.version}</version>
<type>jar</type>
</dependency>
<dependencies>
pom.xml of child B looks like:
<parent>
<groupId>com.example</groupId>
<artifactId>sampleproject</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>
<artifactId>calculator</artifactId>
<packaging>jar</packaging>
When I perform mvn install on parent project. jar files are created in calculator/target/name_of_jar.jar and Gui/target/name_of_jar.jar. But when I check in maven dependencies of Gui project, only directory of calculator is created rather than jar file.
Here is image of maven dependencies in Gui project.
Links I have followed: use-a-class-from-the-other-child-module-in-maven and share-classes-within-modules-in-maven-project

Company Maven POM nested structure

Well I'm working on a company POM Maven structure. I want a structure like the example below. Now I have two questions!
Is this possible for a project to call the parent pom? (considering
that they are all installed in the local Maven repository).
If I adda License block to the Company POM, do I have to add to the
project POM a new License block?
All feedback, suggestions etc are welcome. Thank you in advance!
Company
Company POM
Contains organization information, not to much.
<groupId>org.company</groupId>
<artifactId>company</artifactId>
<version>1</version>
<modules>
<module>company-parent</module>
</modules>
Company Parent POM
Defines for all the projects basic dependencies/plugins (management).
<parent>
<groupId>org.company</groupId>
<artifactId>company</artifactId>
<version>1</version>
</parent>
<artifactId>company-parent</artifactId>
Project
Project POM
Project based organization information (developers etc).
<parent>
<groupId>org.company</groupId>
<artifactId>company-parent</artifactId>
<version>1</version>
</parent>
<groupId>org.company.project</groupId>
<artifactId>project</artifactId>
<version>1.0</version>
<modules>
<module>project-parent</module>
</modules>
Project Parent POM
Defines for the current project used dependencies/plugins (management).
<parent>
<groupId>org.company.project</groupId>
<artifactId>project</artifactId>
<version>1.0</version>
</parent>
<artifactId>project-parent</artifactId>
<modules>
<module>company-module-1</module>
<module>company-module-2</module>
<module>company-module-3</module>
</modules>
Project Module POM's
The modules with each his own buid, dependencies and stuff.
Module 1
<parent>
<groupId>org.company.project</groupId>
<artifactId>project-parent</artifactId>
<version>1.0</version>
</parent>
<artifactId>project-module-1</artifactId>
Module 2
<parent>
<groupId>org.company.project</groupId>
<artifactId>project-parent</artifactId>
<version>1.0</version>
</parent>
<artifactId>project-module-2</artifactId>
Module 3
<parent>
<groupId>org.company.project</groupId>
<artifactId>project-parent</artifactId>
<version>1.0</version>
</parent>
<artifactId>project-module-3</artifactId>

A Maven child project does not "call" a parent project. The POM is declarative, not imperative. Similar to Java class inheritance the child gets the parent's settings and can override them or add new ones. See Maven, POM Reference, 2.2.2. Inheritance.
So, no, you don't have to declare the parent project's licence settings in the child project once again if they are supposed to be the same there.

maven cannot build child module from child directory

I have a maven project with a parent pom and child modules, structured like this:
root/pom.xml
root/parent/pom.xml
root/child1/pom.xml
root/child2/pom.xml
root/child2/child21/pom.xml
Every child pom and the root pom declare root/parent as their parent. child21 has child1 as a dependency.
When I try to execute mvn install from the child1 directory, maven successfully builds and installs the child1 jar. But when i execute mvn install from either the child2 or child21 directories, maven errors out with the following:
[ERROR] Failed to execute goal on project child21: Could not resolve dependencies for project groupId:child21:jar:0.0.1-SNAPSHOT: Failed to collect dependencies at groupId:child1:jar:0.0.1-SNAPSHOT: Failed to read artifact descriptor for groupId:child1:jar:0.0.1-SNAPSHOT: Could not find artifact groupId:parent:pom:0.0.1-SNAPSHOT in jme-repo (http://updates.jmonkeyengine.org/maven/) -> [Help 1]
I have tried googling for what might be wrong with my project structure with no luck. Can anyone help suggest ways to get child21 to build?
My poms contain:
Root
<parent>
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>./parent/pom.xml</relativePath>
</parent>
<artifactId>root</artifactId>
<packaging>pom</packaging>
<modules>
<module>child1</module>
<module>child2</module>
</modules>
Parent
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>pom</packaging>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>groupId</groupId>
<artifactId>child1</artifactId>
<version>${project.version}</version>
</dependency>
...
</dependencies>
<dependencyManagement>
<repositories>
<repository>
<id>jme-repo</id>
<name>JME3 Official Maven Repo</name>
<url>http://updates.jmonkeyengine.org/maven/</url>
</repository>
</repositories>
<build>
<plugins>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.2</version>
<configuration>
<source>${java.source.version}</source>
<target>${java.target.version}</target>
</configuration>
</plugins>
</build>
Child 1
<parent>
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../parent/pom.xml</relativePath>
</parent>
<artifactId>child1</artifactId>
<packaging>jar</packaging>
<dependencies>...</dependencies>
Child 2
<parent>
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../parent/pom.xml</relativePath>
</parent>
<artifactId>child2</artifactId>
<packaging>pom</packaging>
<modules>
<module>child21</module>
</modules>
Child 21
<parent>
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../../parent/pom.xml</relativePath>
</parent>
<artifactId>child21</artifactId>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>groupId</groupId>
<artifactId>child1</artifactId>
</dependency>
</dependencies>
UPDATE WITH SOLUTION
Based on the accepted answer below, I added the parent project as a module of the root pom:
<parent>
<groupId>groupId</groupId>
<artifactId>parent</artifactId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>./parent/pom.xml</relativePath>
</parent>
<artifactId>root</artifactId>
<packaging>pom</packaging>
<modules>
<module>parent</module>
<module>child1</module>
<module>child2</module>
</modules>
Then when developers build the project for the first time, the parent pom is installed. Subsequently they will be able to build the child2 and child21 projects independently.

This is a pretty awkward module structure, but it can still work. The problem is that when you are building module child21 (or child2), the child1 dependency is not a part of the current build, but is looked up in the repository. So is the parent of that dependency, which is the parent module. However, I assume, parent was never installed there - if you run mvn clean install on root, it does not build the parent, which is only referenced in the child modules by relative paths.
To fix it you must first install the parent POM into the repository. Run mvn install in the parent module. After that, build child1 - it will be installed into the repository as well. Now everything what is needed for building module child21 is available in the repository.

Change child21 to inherit from its parent - child2, see below:
<parent>
<groupId>groupId</groupId>
<artifactId>child2</artifactId>
<version>0.0.1-SNAPSHOT</version>
</parent>

Maven module - How to use it as an external lib in an unrelated project

I have set-up a modular project in Maven with a parent project having more than one child projects.
The pom.xml of parent looks as follows -
<project ...>
<modelVersion>4.0.0</modelVersion>
<groupId>com.company</groupId>
<artifactId>parent-app</artifactId>
<version>${parent-application.version}</version>
<packaging>pom</packaging>
<dependencies>...</dependencies>
<properties>
<parent-application.version>1.0</parent-application.version>
</properties>
<modules>
<module>parent-model</module>
<module>parent-masters</module>
<module>parent-web</module>
</modules>
</project>
The pom.xml of child projects looks as follows -
<project ...>
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>com.company</groupId>
<artifactId>parent-app</artifactId>
<version>${parent-application.version}</version>
</parent>
<packaging>jar</packaging>
<artifactId>child-model</artifactId>
<dependencies>...</dependencies>
</project>
Now, I need to use one of the child projects as a lib in a separate unrelated project. The pom of the new project looks like below.
<project ...>
<modelVersion>4.0.0</modelVersion>
<groupId>com.company</groupId>
<artifactId>unrelated-app</artifactId>
<version>1.0</version>
<dependencies>
<dependency>
<groupId>com.company</groupId>
<artifactId>child-model</artifactId>
<version>1.0</version>
</dependency>
</dependencies>
</project>
I keep getting the below error
Illegal character in path at index 57: http://repo.maven.apache.org/maven2/com/company/parent-app/${parent-application.version}/parent-app-${parent-application.version}.pom
The reason seems that I am inheriting the version attribute in child-model from prent-app.
Is there a way to overcome this issue? OR do I need to provide the version for each child module in respective pom.xml and cannot inherit from common parent.
Thanks in advance.

There is a way to overcome this issue with relativePath but I would not recommend it (take a look at this answer)... You should always provide version for parent module. To update all versions of all modules (parent+children) you can use maven-version-plugin or maven-release-plugin.

Maven module hierarchy

I have two modules(maven projects) in parent maven project: android-module and server-module. This two modules uses identical models(POJO - classes). So I want extract models from both modules and make new module in parent project.
So I whant this:
--Project
|--android-module
| -- pom.xml
|--server-module
| -- pom.xml
-- pom.xml
remake to this:
--Project
|--android-module
| -- pom.xml
|--server-module
| -- pom.xml
|--models-module
| -- pom.xml
-- pom.xml
At the same time I want to root pom.xml compiles and build jar from models-module and store jar in my local repository. Then child pom.xml's were taking it from the repository and included in the android and server modules.
Question: How to tell maven to build and store jar in my local repository automatically.
Is it possible? If no - please, give me some ideas.... Thnks

Just run mvn install. This will install the packaged jar into your local repo in ~/.m2
Make sure you run maven from the root pom

The solution is found. It was much easier.
parent pom.xml:
...
<groupId>com.lutshe</groupId>
<artifactId>doiter</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>pom</packaging>
<modules>
<module>doiter-android</module>
<module>doiter-server</module>
<module>doiter-model</module>
</modules>
...
child1 (android project) pom.xml:
...
<parent>
<groupId>com.lutshe</groupId>
<artifactId>doiter</artifactId>
<version>1.0-SNAPSHOT</version>
</parent>
<groupId>com.lutshe</groupId>
<artifactId>doiter-android</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>apk</packaging>
<dependencies>
<dependency>
<groupId>com.lutshe</groupId>
<artifactId>doiter-models</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>
...
child2 (server) pom.xml:
...
<parent>
<groupId>com.lutshe</groupId>
<artifactId>doiter</artifactId>
<version>1.0-SNAPSHOT</version>
</parent>
<groupId>com.doiter.server</groupId>
<artifactId>doiter-server</artifactId>
<version>0.1.0</version>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>com.lutshe</groupId>
<artifactId>doiter-models</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>
...
child3 (shared models) pom.xml:
...
<parent>
<groupId>com.lutshe</groupId>
<artifactId>doiter</artifactId>
<version>1.0-SNAPSHOT</version>
</parent>
<groupId>com.lutshe</groupId>
<artifactId>doiter-model</artifactId>
<version>1.0-SNAPSHOT</version>
...