Gregory Pinkerton

I'm going to have to agree with Kevin on this. Kids compete in wrestling, tae kwon do, kendo, judo, jujutsu, and a number of other 'contact' arts. (I belive in Muay Thai that kids tend to start at an early age, but I may be mistaken as I've no experience with that particular art.)

Dangerous, sure, if there's no one supervising.

(Not having seen a children's mma program in person, it's hard for me to judge. However I doubt someone that's teaching it is going to be let the kids injure themselves that much.)

I'd consider horse riding to be a bit more dangerous as a children's hobby/sport to be honest.

I think it would be worthwhile talking to other children's instructors (in and outside of Aikido) on this discussion. (For those out their teaching children Aikido, do you teach them any joint locks? If so how do you make sure they don't injure each other?)