How to make relationships between users
Using the Entity reference module you can reference all entities (nodes, users, terms etc).

When you use the Entity Reference module you get a field where you can choose one ore many entities to reference. In this case it will be other users you choose (children).

This is the way it could look when you choose children in your user profile.



I suggest you read up on the Module Monday blog post by Jeff Eaton about Entity Reference. to get up to speed on relationships.