I've had hermit crabs and, because they are nocturnal, they are not good pets for children. Any pet a child gets will be taken care of by the adult, just keep that in mind. (Hamsters are also nocturnal, I found out. And don't like being woken up a lot to be played with.)
The hardest part was getting the humidity to stay in the tank, I had to get a piece of plastic to cover the top, and a humidity meter to put inside. You have to check the humidity every day. Plus you need 5-6 inches of sand for them to burrow in when they molt, and sometimes they die during molting.