You must be setting the movable flag on the parent.
Yes, both parent and child must be movable in this case. The user can move the parent object by dragging it, but can also resize the object by clicking and dragging a selection handle. In this situation, the object must change its size interactively, while the user drags the selection handle. This is why I must trap the mouse movement messages for the selection handle, so that I can resize the parent object on each mouse movement message of the selection handle.

I hope this helps explain the situation a little better. So anyway, I'm back to my original problem. On that first mouse movement message, the position returned by event->pos() is in terms of scene coordinates, rather than item coordinates. I can always just ignore this first message, but I would like to understand why this is happening.

Thanks!