Make every children a container to drop dragged element or elements list

Hey. Great pluging. I am just starting with is so I have some hard time now.
I am trying to set all children to be able to put into another children.
For now I can only drag/move elements to elements that have already children. If they dont havy any empty

I cant do that. How can I achive this?

Here is my post on stackoverflow about this issue:
https://stackoverflow.com/questions/27452245/jquery-sortable-how-to-set-all-children-as-a-drop-target