That is difficult, what with the other neighbor seeming to have a relationship with the neighbors yours border....

By living fence, I was thinking about a hedge or something like that. Not always realistic, and of course he didn't get the hint with the other barrier.

I'm not a confrontation person either, but I think the only way you'll know is if you talk to the mother. She may think it's okay because the friend neighbor doesn't have a problem with it. She may feel very badly once it dawns on her. Is there any way you could be outside working when they come by and you could call out something to the effect of, "hi! I'm trying to get my yard manicured and in good shape. Could you please not ride through it?" You could even call out of a window if they can see you. You have every right to ask nicely, and if they don't respond respectfully, that's not on you. Frankly, I think it is very reasonable to not want someone waking or riding through your yard. You arent asking for much. Even if they don't like it, they'll hopefully respect your request.