Wait...they brought an infant with them to go shooting? Good Lord...it's idiots like them that give every other gun owner a bad name!

You mentioned that the property they're on is vacant, right? Can you perhaps block it off somehow? It doesn't do anything now, but it might prevent future visits.

Close enough that you can clearly see the rifle is absolutely absurd...I wish I could offer more approaches with legality, but I have to trust your lawyer friend is more knowledgeable on your state laws than I could hope to be. I wish I could offer you more help...aside from asking them to move away, there's not a lot that can be done for this current "outing."