from the back cover:
All the family are sure that the mischievous Wim Wom will cause trouble for the children if they don't do as they're told, but no one seems clear about what it really looks like. Join the children as they draw, make and model a Wim Wom, using clues from their parents and grandparents. Just as the children decide that the Wim Wom is not actually real, events take a surprising turn. And the children realize you can't always believe everything you are told!