Of the small furry variety - I second a rat. Rats are the most gentle, and smart, and all around friendly of the fuzzy rodent critter pets. My rats were always good at running mazes. It might be a good way to teach kids, not only about caring for a pet, but also about how animals get trained to follow a maze.
Of the larger furry variety - guinea pigs are adorable. While bunnies are sweet, some can kick and scratch when they are angry. Some guinea pigs are skittish as well though. I guess either of these are good but you just want to choose carefully.
Of the aquatic variety. Betta fish or a small freshwater aquarium. Betta's are easy to care for and don't require a lot of space. But if you can, a 20 gallon freshwater tank might be nice. It will not only teach the kids about different types of fish and where they live, but also teach them about the fresh water environment and ecosystem.
I would not recommend a lizard or other reptile due to ceratin health hazards they can pose. They are generally safe, but can carry salmonella and if you do end up with a reptile, you will need to supervise that the children wash their hands thoroughly every time they touch it.