View that shows all children content of parent
It doesn't seem your situation calls for contextual filters, you're not listing anything dependent on circumstances, you aim to list all of the Children, and group them by parent. 

Since it's Children being referencing Parent nodes all you have to do to solve this is to a create Views that list Children, add the Parent field (the entity reference field you have on Child nodes), then under your Format settings set the Grouping field Nr.1 to the Parent entity reference field. This is how you display the children nodes in a sort of a hierarchy based on which Parent they reference. 

You can edit that Field settings if you want to Exclude from display and you can change the Style settings, the grouping output will take that into consideration most of the time.