There's probably absolutely nothing you can do to keep this type of snake out of you yard short paving it and putting up a concrete wall around the property, topped with a trough of flaming gasoline. Luckily, they are completely harmless (unless you're a slug).
Most of the snakes you have to worry about are attracted to rodents and hiding places. Keep as much clutter as possible out of your yard. That will cut down on both snakes and rodents.
Next, teach your children not to touch snakes without calling you first (so you can look them up, since most will be harmless).
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What goes up must come down...unless it exceeds escape velocity.