Good grief... I didn't expect these type of responses from our crew here.
When considering MMA for youngsters look at it like you would any other physical sport. Talk to the coach and see if what he wants out of the youngsters is what you want for your son. There are good and bad coaches in any sport. If the coach appears to align with your ideas for your son then look at the other students/parents to see if they are like-minded as well.
MMA training starts a lot earlier than 10 years old here. That being said it is kept in perspective. The little ones play 'sumo' and work on punching and kicking (not each other but like you'd see in almost any traditional martial arts academy). When they get a bit older they start working on wrestling, no different than if they were just learning to wrestle. The complexity of what they learn is increased with their age and maturity.
If anything I'm a big fan of kids learning MMA young as in a MMA gym there typically isn't some two-year contract which is asinine for young children. Couple that with they get a variety of wrestling, BJJ, Judo, kickboxing, etc... So they have a variety of activities that holds their interest and exposes them to a variety of martial art styles. I'd say start with MMA training and he if prefers one aspect of MMA over MMA in general then switch him to a traditional school of that style.