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Maven multi module project, but children have another parent

I encountered a multi module Maven project where the parent defines some child modules in the pom, but some of the child modules reference another parent. Something like this:
parent pom:
<groupId>test</groupId>
<artifactId>parentAAA</artifactId>
<packaging>pom</packaging>
<version>1.0</version>
<modules>
<module>module1</module>
<module>module2</module>
<module>module3</module>
</modules>
module1 pom:
<parent>
<groupId>test</groupId>
<artifactId>parentBBB</artifactId>
<version>2.0</version>
</parent>
<groupId>test</groupId>
<artifactId>module1</artifactId>
<version>1.0</version>
The project builds with maven 3.6.3 but the versions plugin fails.
Is such a configuration correct? (My assumption is that is isn't)

How can I make an existing maven project as a part of multi module maven project?

My goal is to turn an existing maven project (char-counter) into a module of my multi-module maven project. How can I do this? I use Eclipse 4.16.0.
Here is the structure of my projects.

You can include the sub-modules inside your parent maven project in following way.
Inside your multi-module-project pom.xml
<modules>
<module>char-counter</module>
</modules>
See here: Maven Multiple module

Assuming that you want to convert your char-counter project to multi-module-project.
You need to have following in your multi-module-project pom.
<groupId>com.parent</groupId>
<artifactId>multi-module-project</artifactId>
<version>1.0.0-SNAPSHOT</version>
<packaging>pom</packaging>
<modules>
<module>Anagram<module>
<module>Calculator<module>
</modules
And then in Anagram project's pom :
<parent>
<groupId>com.parent</groupId>
<artifactId>multi-module-project</artifactId>
<version>1.0.0-SNAPSHOT</version>
</parent>
<groupId>com.child</groupId>
<artifactId>Anagram</artifactId>
<version>1.0.0-SNAPSHOT</version>
<packaging>jar</packaging>
And in Calculator project's pom:
<parent>
<groupId>com.parent</groupId>
<artifactId>multi-module-project</artifactId>
<version>1.0.0-SNAPSHOT</version>
</parent>
<groupId>com.child</groupId>
<artifactId>Calcultor</artifactId>
<version>1.0.0-SNAPSHOT</version>
<packaging>jar</packaging>
Also you need to add dependency of one module to other,in which you want to use the code from other module and then you need to deploy the artifact(module) which has dependency included in it and has starting point to run (like main).

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

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 Multi Module Project breaking compile-time class resolution

I've been previously managing a 3-module project as 3 seperate maven projects. As this project has been moving forward, I decided I ought to take advantage of the dependency management of maven2 to streamline integration between these 3 evolving modules.
I defined a super-project that deploys as POM. Some shared dependencies are defined here, and the modules are defined in the POM in the order of dependency from the least dependent module to the most dependent module. Each module has a POM definition back to the parent, and where it applies there are dependencies from one module to the deployed artifact of another module. I'll include possibly worthwhile pom.xml lines at the end.
On to the problem, I setup this project yesterday and was able to get each module building and working on its own. I then come back today to work on one of the modules now that some fresh requirements have come in and all of sudden everything is breaking. I'm editing the projects in Eclipse, and each time I modify a file, it no longer can resolve any of the classes defined within the same project. That is to say if I have a class foo.bar.class1 and it has an object of foo.bar.class2, Eclipse (and the compiler at large) complains that it cannot resolve class foo.bar.class2... Now this is blowing my mind because this other class is in the same project and package. Similar issues are also present for classes not in the same package.
Is there something broken in my maven setup, or does anyone have any idea why these projects can't even resolve classes in the same package??
-::POMs::-
Parent -> /path/to/project/mainApp
<modelVersion>4.0.0</modelVersion>
<groupId>com.moremagic</groupId>
<artifactId>mainApp</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>pom</packaging>
<name>Main App</name>
<modules>
<module>Broker</module>
<module>Soap</module>
<module>UI</module>
</modules>
Broker -> /path/to/project/mainApp/Broker
<parent>
<artifactId>mainApp</artifactId>
<groupId>com.moremagic</groupId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<groupId>com.moremagic</groupId>
<artifactId>Broker</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
Soap -> /path/to/project/mainApp/Soap
<parent>
<artifactId>mainApp</artifactId>
<groupId>com.moremagic</groupId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<groupId>com.moremagic</groupId>
<artifactId>SOAP</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
...
<dependency>
<groupId>com.moremagic</groupId>
<artifactId>Broker</artifactId>
<version>1.0-SNAPSHOT</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
...
UI -> /path/to/project/mainApp/UI
<parent>
<artifactId>mainApp</artifactId>
<groupId>com.moremagic</groupId>
<version>0.0.1-SNAPSHOT</version>
<relativePath>../</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<groupId>com.moremagic</groupId>
<artifactId>UI</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>war</packaging>
...
<dependency>
<groupId>com.moremagic</groupId>
<artifactId>SOAP</artifactId>
<version>1.0-SNAPSHOT</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>com.moremagic</groupId>
<artifactId>Broker</artifactId>
<version>1.0-SNAPSHOT</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
...

It sounds like the problem is with your Eclipse setup and not Maven.
Does mvn compile work from the command-line within your projects? From both the parent project and each individual module (after doing mvn install for the dependencies)?
Are you using a Maven plugin for Eclipse, such as m2eclipse? Check that it is configured to load dependent projects from within Eclipse, rather than looking to the repository ("Enable Workspace Resolution"). What happens if you do Project > Clean to clean out all of the projects?