So the airplane owner is lawyered up, and pretty soon the mom probably will be too. He will argue that the boy had been given neither a key to the airplane nor permission to fly it. The mom will argue otherwise, and her attorney will probably also assert that the inadequately-secured airplane constituted an "attractive nuisance." It will become a big he-said-she-said in court, and the prime evidence will be forensic analysis of the keys (if any) recovered from the crash site and their correspondence (or lack thereof) to the airplanes locks.
It would be pretty sad if it came to this sort of scenario! Take that to it's 'logical' conclusion and you might have a lawyer trying to sue the aircraft manufacturer for
building the aircraft.
This event seems to (tragically) be little more than a teenage show-off and his unthinking friends doing something extremely stupid.