[theamk]

     function make_widget(parent& x):
       w = new Widget()
       x.children.add(w)

RAII is not going to help you here, you need something else (move semantics or refcount-based GC are most common, but other choices exist too).

If this one is too easy, make function return "w" as well, or make it add a widget to two different lists

[zabzonk]

Well, if this is supposed to be something like C++, and if "new" is dynamically allocating memory, then simply don't do that - this kind of thing has been solved years ago.