I wouldn't suggest it, not because of kids in general, most of the time they don't cause an accident, the other guy did, and on bikes a kid can be really hurt or even dead. I suggest you get something else like a go-kart that doesn't go as fast and is a little safer.
Consider the following:
His age - I wouldn't get one for kids younger than high school, personally.
Are they legal in your area? Many areas have banned the mini bikes, especially on the public roads and sidewalks
Are you a pushover parent? Do you ignore your better judgement just because they say " I WANT IT!!!!!' ?
And yes, they are dangerous. Anything, especially with two wheels, that is moving fast involves the potential for getting hurt.