I get that a lot with my kids. If it's rule based, I just reiterate the rule and add something like, "Let his parents take care of him."
If it's a parenting based thing (running around the playground without shoes, etc.) I just say something like, "Honey, I'm not his mother. YOU need to wear shoes."
Recently some kids were playing on top of a box that houses a sprinkler system control at our park. A group of parents who seemed to be attached to the kids were standing right there and not telling them to get down, so I decided to MYOB. Until my kids wanted to climb up there. I pulled them aside and quietly stated that I didn't want them up there because I didn't want them to fall off and since it wasn't really part of the playground equipment, they (my kids) shouldn't be up there anyway.
Apparently, another woman sitting near me (and away from the rest of the group) was one of the kids' parents. She shot me a dirty look and then walked over to the box and yelled at her son to get down. I felt like a jerk at first but then I got over it. I wasn't loudly condemning their parenting, I was just telling my kids why I didn't want them up there. I suppose next time I'll make sure there are no parents around me so I don't accidentally offend anyone.