It seems like you are very protective of your property so you should get a fence. But ya know, I gotta say, I wouldn't think twice about neighbor kids playing ball and using my yard as the outfield. I might even take the opportunity to get to know them rather than harass them about where their balls landed. I honestly think your problem stems from not stating your feelings early on in boundary issue. These kids having been playing like this for years and all of a sudden the property line is an issue. They are children. No one explained to the Dad that their ball seemed to come directly at pregnant you AFTER they were asked to stay in their yard. Now you are the mean neighbors and it's too bad. Put up a 10ft fence, 6 ft will accomplish nothing. Next time you get new neighbors, wether it's you moving or them, make an effort to get to know them immediately. You say they never wave, etc. What is it that you specifically do to invite their friendship?