[FrogInABlender]

Yeah, but you’re absolutely right. I should rein them in more than I do but I guess I don’t want to cause hard feelings in the family for disciplining somebody else’s kids. People get real funny about that kinda thing. But of course there would REALLY be hard feelings if one of my horses did hurt one of their kids, so I guess I’m going to have to be a little tougher the next time.

[AnAmericanMother]

I don't know the parents, but rather than do it in the heat of the moment, I might take them aside at a quiet and neutral time and tell them that you're really worried about the kids playing around the horses, that they don't seem to understand that running up behind them etc. can be dangerous, and you don't want to see them get badly hurt. Ask if they could keep them inside until they're a little older, maybe? or come out with them if they want to come to the barn?

I'm just thinking out loud here, but I think you need to formulate a plan of some sort and follow through. I guarandamtee you the parents will be blaming everybody but themselves (including you) if one of the kids gets hurt. "But you didn't say anything!" "Why didn't you tell us?"