Currently when you store prefab and it sits in the Project folder, you can expand it one time, to get children of the Prefab. If your prefab has more levels parent-children than one, you cant access them in the project panel.
You have to put the prefab into the Hierarchy panel, edit the children and then drag it back and assign in it into the original prefab.
These are additional steps, which could be easily evaded with prefabs being fully expandable to all levels of the children. This gets very cumbersome if you have to edit prefabs like this very often.