Rodents as pets are adorable, especially with long tails, like mice, rats and gerbils.
The only issue I can think of presently with getting a rodent as a pet for a kid is that they can be cannibalistic in certain situations. My friend had three pet rats, and I believe one of them got sick, so the other two ate him. This friend seemed extremely disturbed, and his little brother probably was/would have been too if he saw that. But yeah. I've never experienced this with my pets nor anyone else who I knew had rodents for pets.