cottonmouth snake(s) in yard

Cottonmouths tend to like water (their other name is Water Moccasin), so they are probably coming from a nearby source of water (stream or pond).

But being snakes you are going to have a very hard time preventing them from entering your property if they are so inclined. Snakes can get almost anywhere they want to go.

You're going to need to keep a close eye on the kids, and it would probably be a good idea to clean up any debris or hiding places which are accumulating in the yard or garage or anyplace out doors (since the snakes like to hide in that stuff).

Get a mongoose to replace your pup.