side note: he got a dirtbike for his 7th b'day .. hasn't broken anything on that yet. Got a trick bike last year, no injuries other than a skinned knee or 2.
He gets pissed at me, but I harp at him about wearing a helmet, knee pads, and elbow pads (and dirtbike boots/shin guards, and chest protector when he's on the bike) anytime he's riding anything.
with dirt bikes it's not a question of if, it's a question of when.
my buddy landed a jump on his dirtbike when he was younger and busted his knee up badly... he was wearing full riding gear. his shen bone broke above the riding boot and shot out of his skin, he's missing some cartlage in his knee now, has alot of hardware in there etc. led to an interesting arguement one day about the effectiveness of riding boots... they cut down on ankle injuries, but sure as hell f-up knee's.
my other buddies cousin was recently in a motocross race with a tripple jump... he cased it... again, in full gear. middle of his bike landed square on top of the third jump, broke both his ankles. he's still not walking yet.
i'm all for giving your kid dirtbikes and stuff to ride around, it's what makes kids kids. but don't fool yourself into thinking riding gear saves bones, it only saves lives.