Re: Children transitioning to adult class.
This is not the same, but similar. In bjj and judo, kids have different colored belts then adults.
So kids go white/yellow/green then adults go white/blue/purple/brown/black. In judo the colors are also different. When a kid goes to the adult class, we let them wear their colored belt until they reach the next adult belt (blue for bjj or brown for judo).
I'd probably let the kid wear his belt until his next ranking, then present him with his 'adult' white belt.