If it's a small enough child, and one you are close to, I'd probably go swoop them up and say something like, "Don't worry, I'll save you!".. and run off with them in a joking way. I don't think that most adults who tickle children are perverts or child abusers, they just don't realize that just because the child is laughing doesn't mean they aren't uncomfortable with it.
If it's not someone I'm close to, or a larger child, I'd probably say something like, "Tickler, I don't think Child is really enjoying that. I know he's laughing but he's asked you to stop."