"We All Speak Z" by Melanie Hertling. Illustrated by Heiner Hertling. Hard cover.
We All Speak Z tells the story of restless children who ask their grandfather for a bedtime story. Instead, Grandpa tells the children a secret: when we sleep, we speak the same language of everyone, everywhere – the language of "Z". When the children realize that this means that they can communicate with anyone they can think of, their imaginations run wild with the possibilities.