Kids belts (at least under Gracie Barra) are more time oriented than adult belts. Kids get stripes (the guideline is 1 stripe per month w/ at least 2 classes per week) and the number of stripes between each belt increases. There are also age minimums.
White belt gets 4 stripes, then 1 month after (if kid is at least 4 yrs old)
White/Yellow gets 8 stripes, then 1 month after (if kid is at least 4 yrs old)
Yellow gets 12 stripes, then 1 month after (if kid is at least 10 years old)
Orange belt gets 16 stripes, then 1 month after (if kid is at least 13 years old)
Green belt gets 20 stripes, but when kid turns 16...
Blue Belt!
It's a lot more standardized, so kids get a lot of incremental incentives. The age requirements keep the quality high, so you don't get the TMA thing of 12 yr olds running around with black belts.
My son is stuck with the age thing. He just turned 9, and he's been training since 5-1/2. Can't get his orange until next January. He's so anxious to turn 16. Not to drive, but to get his blue belt. He'll probably get his black belt around 18 or 19. He'll be rediculously good once he starts getting an adult physique!
He'll crush his old man!