Has anyone ever tried the concept of a safeword with their mother?

Not with my mother, but Weasel has known what a "safety word" is (if not necessarily the original context) since she was tiny. If we're having a tickle war or playfight and she says the safety word, the silliness stops Right Then. It works the other way round, too -- if I or the DH say it, _she_ stops.

I can't see us abandoning the safety word altogether, although we may not use it just for playfights when she's an adult.