I ride both ATV's and Dirtbikes. Both dangerous as $#@. If you want a safe alternative take up badmitton or tennis. Motorsports are gonna be dangerous. For me the dirtbikes are more fun. I mastered the basic ATV riding after a few rides and decided that it would be easy to get seriously hurt racing them. So back to the dirtbikes and I just play on the quad once and a while.
12 year old I would put on a dirtbike first. Probably gonna take some soil samples anyway and everyone on here is correct when they say crashing a dirt bike has less chance of serious injury than a quad. With dirtbikes you crash more often, just not as serious as the quad hopefully.
Just my two cents trying to get the 15 post mark.