I think it's fine as long as you set expectations appropriately. Young kids are not going to learn aikido, but they can learn the fundamental building blocks of movement that ultimately add up to aikido. Even older kids are not going to learn more than a few techniques. They're not going to want to train if it's not fun pretty much all the time -- don't expect them to. If you're okay with letting kids be kids, and not getting too stressed about whether they're doing "O Sensei's aikido" (they're not), it can be a good time for all concerned.
What did you mean by young and by learning aikido?
I've worked with some 8 - 12 year olds that get aikido as well as talented early kyu adults. The really talented kids tend to be 1st or 2nd kyu by 15 and black belts by 18.
Most kids don't get it at all, but that's not much different from the adult classes. Most adults quit before they really learn aikido. Just my observation of course.