I'm not sure which design decision would be best - Idea 2 and Idea 3 seem pretty much the same, with the small difference that Idea 2 would make this message usable from other places as well. If you would need the functionality only in Parent, then I suppose Idea 3 would be most "encapsulating".

And for you SDK query about the data cast - this should do the trick. Usually when doing such node data queries, you should always check the type with GetType(), but since you create the object on the line above, the check would be redundant.

BaseObject* child = BaseObject::Alloc(Ochild);
if (child) {
  Child* castedChild = child->GetNodeData<Child>();
  castedChild->CreateNulls(childNullsCount, childNullsShape);
}