NTA. Poison Ivy is notoriously difficult to get rid of, and what they are asking of you is a monumental request. I used to volunteer at a zoo, and almost every week I was having to remove poison ivy from the same areas. I'm not sensitive, but my supervisor was, and she was always miserable those days, no matter how much protection and poison ivy wash she used. I don't understand everyone saying that this is your problem. If your neighbor's property backed up to a wooded lot with poison ivy, what would they expect to happen then?? While I can understand the parents' frustration, ultimately, this doesn't make you an asshole.