Re: Is aikido suitable for children.
I've taught children's classes a bit. They are quite fun and VERY good for the children. in my kid's classes, we practice only basic techniques, repeatedly. I try to make it fun and interesting by playing a lot of games relating to aikido and aikido movement.
Aikido can teach children things like discipline and body/spacial awareness. It can really help kids with attention problems (such as ADD & ADHD), by giving them activity and then requiring them to be calm and pay attention for a little while.
I began Aikido at age 5 (I'm now 19). I am so glad that I did. When I was a kid, aikido afftected the way I thought, moved and made observations; it still does.
Children will often see things in Aikido that adults will not. (It's that whole young, fresh, innocent, open mind concept.)
As for mixed classes, they can be great, if done correctly. Generally beginners should not work with children, because they do not usually have enough control over their own bodies and strength. Advanced students are usually quite capable of practicing with kids, and can even learn from them.
In my dojo, we used to have something we called "family class." This was a class where parents could practice with their children, siblings could practice together, etc. This is the only time we let beginners practice with children. My dojo has many families who practice together, and it's great.