There is a way to draw to a parent panel when it has other child panels. You can "lock" the child panel so it can't be a target of the added/drawn elements.

Anyway, I've seen that Blend keeps the selection as parent of the newly created elements. So if you have a Grid selected, and that grid has another panel covering it entirely, any new added/dreawn elements are still added to the selected Grid. If the selected element doesn't allow adding children, then its parent will be the target of the newly added elements. We should do something like that.