One super-duper-goes straight to the hindbrain keyword that the parents teach the kids and then do not use except in dire need.
A word that means STOP, but cannot be ignored the way kids tend to do with everyday situations when they are pushing the boundaries.
We took the word "Hold" from the SCA and use it on board. If our two year old hears it, he will freeze on the spot, check his immediate surroundings and sit down in the lowest spot that is nearby (usually cockpit floor).
We have only needed to use it once in earnest, but were certainly glad it was there.
The other advice here is also very good and should be taken on board.