The Giggler Treatment (Rover Adventures, #1)

Mr Mack's dog Rover sells is own poo to the gigglers - small creatures who take revenge on any adult who treats children unfairly by making the unsuspecting adults step in poo. When the gigglers set out to exact punishment on Mr Mack, Rover knows he doesn't deserve it, and the race is on to get to him before he takes that fatal step.