I was tickled mercilessly growing up until I was about 8 or 9 years old. My dad had built us a bunk bed and I was sitting on the top bunk when he came in to tickle me. I ended up spontaneously kicking him in the face (I honestly didn't mean it, but he went after my feet). I can tell you, I was never tickled by him again after that. It took some of my friends in high school learning the same lesson the same way, despite having been told that I flail and thrash.
Depending on the age of the child, if someone is in actual danger of them thrashing to get away and getting hurt, step in on those grounds. If someone calls you on it, again, white lie. "One of my friends actually hurt someone pretty badly trying to get away from being tickled. I'd hate to see you kicked in the face over this, and it doesn't look like kid is enjoying it too much any more."