in our dojo we have different kid classes, but not different kid ranks. They have to do the same test in order to get the same belt. A kid yonkyu would have to know the same techniques as an adult yonkyu.
There is a difference about wrist locks, but that's all of it. There is also a difference about ukemi - no one would really slam a kid into the tatami, but we ARE teaching them breakfalls. I taught them even the "falling leave" (I don't know if you call it like that in English, it's called "la feuille qui tombe" in French), and now all kids are able to do that one, but nearly none of the adults. In a neighbouring dojo I saw during a seminar a yellow belt, maybe nine years or ten years old, performing ushiro otoshi on irimi nage omote. That was impressing.
Kids need longer to get to a rank, so they get stripes in between. My daughters have, after two years and a half of aikido, two yellow stripes on their white belt. There are some limits, I think for shodan you need to be at least sixteen, and for 2nd kyu at least 12. But I'm not sure - there are so few kids ever getting to this level that awarding high ranks too early is not really an issue.