I think the point isn't whether martial arts can't be beneficial to kids. The point is, in many programs, the kids wear the same belts as the adults, which in the minds of some, cheapens the achievement of any given rank. Because the truth is, kids come into martial arts with a wide variety of abilities and at different developmental levels. And, at almost every level from white belt, to green belt all the way up to high brown, an 8 or 10 year old won't be at the same level as a person of the same rank who is 15 or older.
At my dojang, there are kids only classes which most of the kids attend, but kids are also allowed to attend adult classes, which are labeled "family" classes. I will be honest. I don't mind training with teenagers. Some of the teenagers at my school remind me a little of me when I was 15 or 16. ,But I don't like training with kids younger than 12. It makes me feel ridiculous, especially at this juncture where I am junior to almost everyone. I look forward to the day when I can attend more of the advanced classes where the kids are either working at a much higher level, are older than 12, or most likely both.