@Butitsnotfunnyisititsserious
Out of interest, why do people feel entitled to feed someone else's horse?
What is it about horses, that gives a random stranger, the right to feed them?
If you want your child to feed / pet an animal, take them to your local petting zoo.
I think this is part of the problem, though. To many of us, and to our kids, our only encounters with animals apart from cats and dogs is at a petting zoo - where we are frequently actively encouraged to pet and feed them. And of course animals in a field are different, but if you've been "trained" by petting zoos that it's OK to feed goats, pugs, etc - it's easy to see why people think it's fine outside a petting zoo, too. I've learned a lot on this thread that I didn't know before.
Someone said here that horsey people like horses because they are not great with people. I am in a people-facing role where my tasks include keeping many, many people safe from their surroundings - and keeping their surroundings safe from the people. Sometimes yelling at them helps. But it more often makes them defensive and more inclined to carry on with the behaviour you want to stop. People also get to a point where they don't notice signs any more, but there's tricks that help, eg handwritten signs, and signs that have someone's name on them, so people connect the sign as a real person asking them not to do something, not a standard bit of bureaucracy that doesn't matter. (Eg not "Feeding the horses is forbidden", but "Molly the horse will get sick and may die if you feed her anything extra, even grass. Please help us keep her safe! Signed Jo and Freda, from the farm". It's amazing how much difference it makes.)
In the end, you can yell all you like, and you may be absolutely correct in what you are saying, but when it comes to matters of life and death, you have to work with how people are, not how you would like them to be. It's not enough to be right, you need to be effective, and just yelling at people that they are entitled for being interested in your horses won't ultimately keep those horses safe.