I would be inclined to put a non-see-through/blockade fence up on their side of the back yard so they couldn't see the pool, etc. I would lay down rules for playing in my yard and if the girl behaved badly, she could go sit in her yard alone as a "time out". But I'd also find an inexpensive YMCA or something day camp for my son to attend for a week or two to give me a bit of time off with watching kid(s).