This is always tough. I'm guessing you don't feel comfortable talking to the mom about it? Some people think nothing of this kind of thing and it might not even cross their minds just how rude it is.

I live in an area of the country where pretty much all backyards are fenced with six foot fences, and at one point we had a neighbor who thought nothing of hopping the fence- generally because a ball got tossed over. Apparently this was typical where he was from? But it sure bothered me. Someone else must have told him to not do it, because he stopped after a week or two.

Could you plant a living fence of some kind?

How about a cute/decorative no trespassing sign? Something that might give them the hint nicely? You could take it down once they get the idea? Maybe a sign that says something like "Private Property - Please no through traffic." Or something like that? I have a no soliciting sign on the front door, and painted it myself so it seemed less like a directive and more like a decorative, but pointed, sign.

Not necessarily the best idea, but I've known people who got those motion sensor sprinklers that are made to keep deer out of your yard. Unfortunately, the kids might like it or you might get one of your other neighbors by accident.