An emergency meeting of the Mortimer Square Garden Committee has been convened to discuss a most alarming matter: Someone has been digging in the garden and making off with buckets of dirt. Miss Angela Chesney is sure that a gang of boys from run-down Catford Street is to blame. But Angela's sister, Olivia, isn't so sure. Olivia has always wondered why the neighborhood children-the "sparrows" she sometimes watches from the window of her house-are kept out of the private garden. Don't they have a right to enjoy the place, too? But neither Angela nor Olivia has any idea what sent the neighborhood waif Lovejoy Mason and her few friends