If your minor child steals your car, you are still liable for the accident.
This kid was not a stranger off the street, nor a natural child. When you take in people to play family, you start sharing liability.
Was the gun stolen or not? Simple question. Simple answer. Yes or No.
If your answer is Yes, then you are admitting that the owner of the gun LACKED THE REQUISITE INTENT.
Was what the gun owner did stupid?
Maybe.
But we don’t put people in jail for being stupid.