here's what i would do. go confront the mother, face to face, one last time. tell her, in a way she WILL understand, that if her son EVER comes in your yard or harms your dogs (or attempts to) again you will call animal control and the police and her son will be taken away for animal abuse and parental neglect. (whether it's true where you live or not is beyond the point, but some FEAR into her by God!)
after that, go about your normal day. if it happens again, grab the kid by the arm, bring him to his mother and tell her that this was her last chance and your calling the cops and animal control NOW. go inside, get your phone and make the calls where she see's you doing it. when you call the cops tell them that you would like to file a harassment charge against your neighbors and their son and you would also like to file a trespassing charge against them. the cops will have to come because you want to file the charges and they'll need to talk to the neighbors. call animal control next...tell them that a 4 yr old boy is abusing your dogs and could they (AC) come by and talk to the mother about this (also that he's doing the same to the neighbors dogs). THEN, call Dept of Family Services. tell them that a 4 yr old child is being allowed to run loose into other peoples yards, torment/torture/beat other people's dogs and the parents are being neglectful. have the neighbor call Dept of Family Services as well....they'll have 2 calls saying the same thing and will have to send someone to investigate the childs living situation.
if you have a video camera, take it with you if you think he'll be outside. the second he makes a move towards your dogs, turn the camera on and get it on tape. then you will have indisputable proof of what's going on and SOMEONE will get it then.
