I'm a little surprised by the number of people that are really bothered by kids on their property. I live in a rural area of the state. My neighbors hike on our acreage and there are probably others that I don't know about. I don't remember them asking either. Kids cross into my property while playing too. I don't have a fence and fences aren't commonplace here.
People, especially kids, coming unsupervised onto a property with animals--especially large dogs and/or livestock--are a liability issue. Heck, a few species of livestock can kill or severely injure an adult without intending to, much of less a child.
Also, if the OP's fence in not in great shape the kids could hurt themselves hopping it, which could be another liability issue. Aside from all that it's extremely rude, IMHO, to hop people's fences without asking or hike their property without permission, I don't care if you have 1/4 acre or a 100 acres.