Agree.. The garter snake idea is not bad but keep in mind they are a musking snake. If it is being handled and decides its not happy I doubt the kids will like it much when they get to smell its stinky musk. Or when their fingers & hands smell funny after they have touched it.
That said. Yes any snake can "pee" on you to show its not happy but musking snakes are much stinkier then any others in this situation.
A corn would probably be best. Much more affordable and less likely to develop any health problems due to improper housing since they do not require the strict high heat & humidity of some other snakes.