There are actually a few reasons that kids get different belts. Here are a few:
- You are supposed to have the body of an adult before you are able to get your blue belt or anything higher.
- Kids classes are often shorter, and since BJJ promotes based on time, there needs to be a way to make up for the extra time that they would have to put in if they were an adult
- Kids classes often have more "messing around" than adult classes, so less technique is sometimes learned.
- It gives an easy chance for kids to show their exact skill level by the time they turn 16.
- Kids are often more likely to lose interest in the martial art if they are not frequently receiving promotions, that's why the first two belts (white and grey-white) take less that 6 months each, and then they start to take about a year each.