Mia has a secret word, let's say it is Tulip. If someone she is not expecting shows up to pick her up from somewhere and says "Hey, your mom asked me to come get you" she knows to ask them for the secret word and run screaming in the other direction if they can't tell her it is Tulip. (Clearly this is for someone she knows, for strangers she knows to run screaming immediately.)
Owen knows the secret word too, but doesn't totally understand its application. So when we are playing Tickle or Wrestle or Chase or Trap Owen (his favorite) and Owen is done playing the game, he shouts out Tulip! Tulip!
So yes, my three-year-old has a safe word. As you can imagine, I am quite proud.