There are actually a few reasons that kids get different belts. Here are a few:

1. You are supposed to have the body of an adult before you are able to get your blue belt or anything higher.
2. 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
3. Kids classes often have more "messing around" than adult classes, so less technique is sometimes learned.
4. It gives an easy chance for kids to show their exact skill level by the time they turn 16.

There are actually a few reasons that kids get different belts. Here are a few:

1. You are supposed to have the body of an adult before you are able to get your blue belt or anything higher.
2. 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
3. Kids classes often have more "messing around" than adult classes, so less technique is sometimes learned.
4. It gives an easy chance for kids to show their exact skill level by the time they turn 16.
5. 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.