From the picture shown here they weren't Putting on any protective which is not appropriate for them at their age to have engaged in such an exercise. Any damages directly on their body remains there forever because they are still growing and immature.
If they deem such sports valid, then there should be strong policies good enough to protect the children who take part in such exercise but I don't think such violent sports is appropriate at such age.
What would be a appropriate protection at their age? Head gear? Then it wont be a fight under mma rules, as you wont be able to perform some techniques. From the picture, it looks like they are more teenagers, than little kids.
What about these kids then? No head gear also, but they have a full contact fight.
Dont even try to search for kids Muay Thai tournament pictues
Those little kids also dont wear any head protection, but from very childhood they learn how to land elbows, that opens soft skin easily and make deep cuts. When you say that there must be a special policy for kids, sports and violence, but like it was said above, how come you know you are ready and learning right, if you havent tested yourself in fight? Those fights turn children into grown ups. Or you suggest for kids to start training combat sports only when they are 18?