Welcome to the forum. There are many kinds of snakes that can be housed in a 40 gallon. Your first task is to narrow it down and decide what you want. Then visit the"Care Guide" page and start from there. Since you have small children I recommend either a Corn Snake or Ball Python because they are known for being very docile. It's wonderful that you have added yourself and your family to the community of reptile enthusiasts. People here are more than happy to answer any questions you have. And again, Welcome!