Tell them to open their mouths as big as possible and while doing that they have to whisper. It was hard to sing in a whisper while mouths are open wide but they loved it. So fun to see their faces too...i'm sure mine looked just as comical! :) This is a great one to add to your "sing like a..." list. You could pretend you're all wide mouth frogs who can only whisper. Start by "croaking" the song, then sing (wide mouth whisper) the words.