There are many small snakes that WITH supervision make good pets for kids, but many should never be around children. Just as with children an adult can also be injured or killed. If your snake will become large, be sure you have enough friends to help. Baby snakes are just like puppies. Some will stay small others will become very large.
Snakes also grow fast.
Make sure you the room now, for when it becomes full grown. Well let just see what it will cost. Of course these are rough numbers. Much depends on what kind of snake, and where you live. Also many times you can get used items.