Let Parent View child's result
Download and install the Entity Reference module
In parent user fields, add a field with Entity Reference and a widget of Auto complete (tag style) 


For example: I named mine lalala (click picture below to enlarge)




For Target Type, select User 
Number of Values: Unlimited (as parent could have more than 1 child)





Then go to the parents account and start typing child's account user name, you will see the name in the drop down, select it and save.


Use a comma , to add other child account. (parent has more than 1 child)

 

In your view


contextual filter of User id (uid) (which uses the parent's id)
add a relationship of lalala Referenced Entity


If not, try Referencing entity, but if my memory is correct it's Referenced