Re: Children transitioning to adult class.
A couple of quick replies:
1. In our dojo, they wear their colored belts until they test for their first "adult ranking", then conform to adult standards. The situation is explained to them upfront as they begin to start adult classes. None of our students making that change complained. Nor did the adults.
2. To keep students from leaving as they hit teen years, we started a teen class two years ago. It's run like adult class, essentially, but without the need to "be" adults yet. It's worked very well, and has kept 50% of our teens going into adult class.