I have to agree with rats...I've had quite a few of just about every type of rodent commonly kept as a pet, and rats have been the best by far. An adult rat is too big for children to get their hands around enough to hurt them, and their temperament is much better-suited for young handlers. In my experience, gerbils tend to be extremely hyper and hamsters have always needed a lot of work to get past being skittish and/or mean. Rats also don't need training like a puppy will, and they're much less likely to scratch or bite than the average kitten.
Alternatively, if you're looking for more of a "display pet" to teach them responsibility and offer a learning opportunity, I invite you to check out my hub on keeping bugs as pets. I also have one on spiders, but spiders take a bit more work and commitment so I would not really recommend them for children.