I've seen kids training while they're young but not into this kind of sport. Whoever gave a permission for this are to be blamed and the first people that I can think of are these kids' parents'.
Sportsbook shouldn't tolerate this and shouldn't put it into their books neither give any of these kids a sponsor to discourage that there's no sense doing this whether for passion or for money.
Stop. You want to blame parents because the have send their kids into such kind of sport? The whole idea of the sport these kids are in is fighting. I see nothing wrong if kids train martial arts, nothing wrong if they compete. After all, there is a referee in the cage that will save them from serious danger. Bleeding nose, black eyes, that is ok if your kid trains martial arts. If you have opposite opinion, then at what age should they start training? In my town, martial arts welcome kids starting from 7 years. They dont fight each other at such age, but at 10+ they already participate in competitions. And did you know, that in many martial arts, you cant get a black belt, if you dont participate in tournaments and win them.
But what I dont support is betting on junior martial arts competitions. Whenever someone looses a bet, he will always blame a fighter he has bet on. That is wrong, as they are young, inexperienced amateurs.