Is there anyway you can keep the kittens indoors till they grow? Not knowing your living situation, forgive me if that was just a stupid question. This is very sad, but instinct is in play here, so keeping them in a safer environment temporarily may be needed,till they mature enough to follow instinct.
Personally, we never let our cats out, due to one sad incident, it wandered and was ran over, it was horrible to walk and find it all smashed. We no longer have cats, but 2 large dogs, we are on 2 acres, and I have a large area fenced in that I can open kitchen door to let them out and potty, then come back indoors.
My old dog no longer attempts to climb, but the 2yr old (who is larger) cannot be trusted, they all learn the "houdini tricks" so she is not left unattended.
As parents to little kids, we also need to apply that parenting to our pets (children with fur) being responsible about their whereabouts and protection from the "bullies".
It is sticky situation, I'm sure, but you may have to talk to the owners.
Not having cats for so long, I'm not up on many things, but isn't there some products/deterrents you can use on your property to scare the "outsiders" ? Maybe your vet or a local pet supply store can make some suggestions.
Your cats should not have to be victims,and unfortunately it it seems you are the one that will need to take care of this, sounds like the neighbors won't especially if they are unaware of this stuff. It's sad if they really do not care about the safety and health of their own cats.
My heartfelt feeling towards owners like that is if they are not going to be responsible why even take on a pet, or even have kids, for that matter.
Behooves me. I understand the BB gun thing, LOL!! I've been using this loud safety alarm keychain thingy, to scare off groundhogs,rabbits,cats,fox,etc. from wandering inside my fenced area. I had to call animal control once, a nasty groundhog took refuge under my backdoor (open) steps,he wouldn't scare out, just growled showing some fangs.
2 things would have happened if I didn't take action, 1) dogs would of mauled it and 2)dogs would have injuries too.
You may have to resort to calling animal control, sometimes over the phone they can offer useful info.
Take good care and stay safe,
DE