Be the Boo Radley of the neighborhood
The make 'em think YOU’RE crazy idea can work too.
My DH is known to the neighbor kids as “that crazy-a** guy at the farm” because the few times they’ve ventured onto the property without asking he went nutso on them - yelling, screaming, threatening (you can threaten if you make it conditional - I will kill you IF you come back here) all while waving his arms and getting right up close. They probably did think he’d kill them with his bare hands.
Edited to add that the ‘kids’ in this case were 17 and 18. You’re right, this wouldn’t be the way to deal with 8 year olds. He did not actually say he would kill them ( I think it was more like you will regret ever setting foot here again), that was MY example of a conditional threat. In NH, the live free or die state, you are allowed to make conditional threats concerning people coming onto your property. But that may not be the case everywhere.
While he was actually fairly angry, he was also totally acting, to make them think it would be SO not worth it to come back.
Your dad may be the one who has to do this, but don’t discount the crazy lady on the farm either.
And call the police each and every time you find them or their dogs on the property.
You will not be popular, but you and your horses will be safer.