First, I must apologise for taking an unconscionably long time to reply to your email. I can only plead in excuse that we have had some traumatic family problems in the last few months which have occupied all my attention and only now have I been able to return to my responsibilities.
If the child were 25 and perfectly capable of taking care of himself and living away from home, would you think that the action of his parents in forcibly forcing him home would be defensible from a libertarian point of view? No? Then what about a 20 year old in similar circumstances? No? I’m sure you can see where I’m going with this. Age is irrelevant except insofar as it constitutes prima facie evidence of inability to care for oneself. So long as you grant the (counter-intuitive) point about the 12 year old being able to take perfectly good care of himself, you cannot justify, on libertarian principles, the (understandable but indefensible) actions of his parents in forcibly moving him home.
I suspect that in a real-life situation, most parents would do as you suggest and take their chances in a libertarian court of establishing the real point at issue which is whether or not, in fact, a 12 year old is perfectly capable of taking care of himself for the rest of his life.
Once again, apologies for the delay in responding to your query.