A cat is always a good choice. A dog is too, well depending on the type of dog. You don't want an agressive dog or anything that will grow quickly to be bigger than your child...
I notice a lot of people are saying fish. But really, if you were a child you would want something to play with, not a fish which will just swim about a bit and entertain you for maybe 10mins and thats that. A dog is great as you can then encourage your kid to go on walks (to walk the dog), and they are very playful.