(my daughter is in the process of potty training, but still wears pull-ups at night when this conversation happens as I am getting her ready for bed.)

Mama:  Come on, you have to wear a diaper at night.
three-year-old girl:  NOOOOOOOOOOO!
Mama:  Okay, but then you can't come see me tonight when we're all asleep.
three-year-old girl:  (only slightly mocking) What are you gonna do about it?  Put up a sign?