I believe children can be taught and that they can learn aikido, you just need to know how to teach children. Children learn differently than adults but it doesn't mean they can't learn.
My sensei, has been teaching children for years and when I train with those who started at a young age and now can train with the adults (usually in their early teens) they have incredibly solid basics that I don't see in most in adults that I have trained with. The reason is that their curriculum is based on the fundamentals and basics of aikido. We make if fun for them but my sensei doesn't teach down to them such as only teaching games.
I have seen young children learn to improve their attention span. I have seen them gain confidence as they move up in rank. I have seen them learn to control themselves and listen and respect their seniors. Our aikido program provides this without lectures but with simple aikido instruction. I think we need more of this in the world, not less
If you want to know more, check out the article on www.aikidoonline.com
written by Peter Bernath about teaching children aikido.