When the are very bad, or when you’re feeling particularly in the mood, take out a red and a blue marble. let them see both. put one in one hand, one in the other. Tell them to pick a fist. Open your hand to reveal a red punishment marble.
Then ask them, very slowly, if they trust that in the other hand is a blue marble. Tell them if its blue, they cancel out. just a nice day. If its red, they get double punishment. Give them the option of looking.
Its entirely up to you if you want to employ sleight here. I prefer games where the opposing party relies on fairness as an article of faith rather than a known entity. because then they arent wagering against the existence of blue marbles. They’re wagering against the existence of my mercy.