Leave the region play abit of the story then come back should be a child then. If you've done the story, just leave the region and wait a couple of in-game days and the transition should happen.
If not, just wait; the only thing for this is patience and away time from the region. If it still persists after a week or two in-game you may have a bug on your hands, but its not one I've heard of, you could try to save and re-load or reboot your Xbox if none of this works I really don't know what will.
Good luck.