How about a guinea pig (cavy)? I never would have given them a thought, but we cared for the school piggie during some weekends and vacations, and long story, sort of inherited him as a full time pet. He was great- cuddly, content to sit in laps or to be dressed up and paraded around (DD did a lot of pet clothing design and sewing at the time, and he was very patient). He was fun to feed (loved dandelions in particular, which we have in abundance). The kids made all sorts of habitats and mazes for him, both inside and out (and didn't seem to mind that he usually just sat in one place).
The main downside was cleaning the cage, but we managed to get by weekly, and it's easy for kids to learn to do it, with some help. I also think they are social animals, and books suggest they do better with a buddy piggie, but ours came to us solo and seemed ok.
