Well, I have a ferret, rats and snakes.
The ferret was found and we agreed to keep it as the owner couldn't be found. We do plan on getting him neutered and getting him a friend, but are slightly concerned he is too old and set in his ways, as he is huge. He does absolutely stink. His (very large) cage is in the garage and he comes out everday for a play. He did nip a bit when we first got him, but not any more. We just used to say 'ouch' loudly when he nipped and put him back down each time. He soon realised that nipping = no play time.
Snakes can be good pets, but they tolerate being handled rather than like it. They are easy to keep once you have the set up and know how, but take a bit of time, money and research to do this. Great if you go away as they only need feeding once a week or so, so someone just needs to check on them and clean the water bowl. Mine are a hobby rather than pets.
Rats are wonderful pets. Not to fast for children, well bred ones don't bite and they are super friendly and intelligent. They can be trained to do all sorts, including come to their name. They do need larger than you'd expect cages and time to come out and play/attention each day, plus a good diet (not commercial rat food) but they are very, very worth it. They can smell, but you can litter train them and does smell much less than bucks.
Out of the three, I would choose rats as the best pet for children.