Our safe word has become sacred, because there's a lot of benefit to having it, so yielding to it works.
I did date a woman who had two daughters.. she needed a safe word in their dynamic but tried introducing it too late in the game.
I think you have to start this one early.
Kid 1 does something too repetitive.
Kid 2: "badingo!"
Kid 1 stops, but does something else too repetitive.
Kid 2: "badingo!"
Kid 1: "You're saying badingo too many times. Badingo!"
Kid 2: "badingo badingo badingo badingo badingo!"
Kid 1: "badingo times infinity!"
Kid 2: MOM!
Actually that kind of discussion impresses me because it shows they comprehend infinite loops and recursion at some level.