It is OK. Junior black belts are a thing now. As long as the school maintains its standards for adults, I don't see a problem with it. Sort of inevitable, really, that if a school starts kids out as young as 6 or 7, by the time they turn 13, some of them will be black belts.
I don't know man, take Brazilian Jiu-Jitsu for example. We have kids whom their parents live vicariously through them, so they go crazy and make their kids start training at age 4. At age 6, they start getting serious with competitions = 5-6 days of training at 1-2 hours per day + 3 hours of competition training on Sat or Sun (sometimes both). 1 of our 12 year olds, can fill up 2000 s/q feet of mat space with his medals, belts, sword trophies and trophy trophies (photo op).
Many of these kids train from age 4-16 = 12 years....earning only kids' colored belts. At age 16 is when they get adult rankings....which they have to prove themselves first in adult classes for many months (by tapping lots of adults)....then they earn a BLUE belt at 16.....(white, blue, purple, brown, black). These 14-16 year olds will usually decimate all of the Adult White belts to some of the Adult Blues.....up to 30-40 lbs heavier. Like if a new guy just walked off the streets to try it out, average adult in good shape.....will get tapped out every 60-120 seconds if these kids wanted to.