Yeah, it sounds like it might be a great case to run up the flagpole to SCOTUS for an official ruling, since it crosses state lines.
Like …okay the child is behaving in a way inconsistent with State B Law, but they’re not in State B. That happens all the damn time, every day, with vice laws, weed laws, gun laws, etc.
Also, presumably, if the child moved out of the country, State B would be completely unable to enforce its laws in country B. So there’s a limit to this enforcement, but where is it?
It would absolutely be Dr. Spaceman!