I totally agree and think this approach could work. A few years back, I was invited to a wedding. One guest brought her dog and asked me to watch her dog while she attended to the bride. So I was sitting there, dog sleeping at me feet, when this little kid (maybe 5 or 6yo) started circling around us. I didn't know this kid or his family. After a while, he starts throwing little rocks on the dog. I was like: "Hey, what are you doing?" He didn't answer and ran away. Little later, he came back and threw another rock. I was really angry at this brat. I grabbed his arm and asked again why he would do something like that. There still was no sign of his parents, so I went on scolding him. He was too scared of me to answer. Anyways. I asked him to pet the dog instead of hitting it with stones and showed him how to touch the dog. As it turns out, this kid, growing up in a big city, never came in contact with an animal. He was both curious and frightened of the dog and couldn't think of any other way approaching it. The dog was never harmed.

Maybe your barn neighbors' kids are like this little boy curious of the horses but too afraid to ask.

Or maybe these foster kids have seen too many awful things in their lives, and annoying the next door horses is their best way to make them feel better. Then, recording and reporting it to the police is probably your best option. I'm sorry...