Only people who are unaware of the complete lack of security at most GA
airports are wondering....

Really, you're becoming as predictable as the tide with your asinine &
uneducated comments...

It has been widely reported (if you were paying attention) that the
airplane belonged to one of the boys' fathers. That being the case it's
probably safe to assume that they had access to the aircraft. The OP
said they'd stolen the keys (that should've been your first clue)