Re: Children transitioning to adult class.
Just my thoughts:
kids will leave in their teens no matter what. That is true of all activities not just Aikido.
In terms of those who stay there needs to be some method of transition, over the past couple of years we have adopted the following system.
6 - 12 years solid belt colours
12-18 belts with horizontal white stripe
also only 6 kyu grades but each is split into three parts so that grades go mukyu + 1 stripe, + 2 stripes, 6th kyu and so on.
At age 18 they have to sit their current grade again.