Personally, when choosing a pet for kids, one thing you must into consideration:
1) If you are not allowing the kid to choose their first choice, are you certain they will maintain their interest for long？
2）Besides that, as a parent, will you willing to step in to take over the responsibility if your kid lose interest with the pet? If yes, then also put your thought for ideal pet into consideration as well.
3) Most of the warm-blooded animals will required some grooming such as clipping nails and bath. This is something that kids might not able to do, will you afford to help them to perform the tasks or pay to send to a grooming center?

If you have not figured these two, maybe can just to a fish shop to buy those feeding fish or fighting fish or guppy to test water. At least their life span is shorter and more durable for starter, and cost wise is slightly cheaper compared to other pets. (Maybe apart insect)