Did you ever find a solution? I would like to know too.
EDIT: At the moment I clear the box from all children after the point where I want to add the specific child. And then I recreate the children I had to delete. Given that widgets cannot be destroyed during a level, with this workaround I get quite an enormous number of unnecessary objects stacking in memory.
I would really like to be able to organize children via indexes in blueprint, exactly like you can do inside the editor with the arrows or dragging, or in c++.