This evening, well after the girls should have been asleep, I kept hearing little feet shuffling across the floor. After quite a while, I went to the stairwell and said, "I shouldn't hear anymore sounds up there. Every time I hear sounds coming from up there your bedtime tomorrow gets earlier and earlier because I know you're not getting the sleep you should be getting tonight."
It was then that I heard the last quiet shuffle and a tiny voice (Avery's) called down quietly, "I'll shush." And sure enough, she did.