The Reefs of Earth by R.A. Lafferty
Berkley Medallion Books, 1968
Price I paid: 90¢
A PLAGUE OF DEMONS
—that’s what the people of Lost Haven called the six children (seven, if you counted Bad John) of the Dulanty Family. They looked like normal Earth children…except when they flicked their ears like animals, or made their eyes glow with a green fire…and if you looked at them sideways they did look strangely like nightmarish gargoyles.
The truth is: these children are Pucas, aliens from a strange planet. And they have taken it upon themselves to reduce the world to a population of six (seven, if you count Bad John). Wishing will make it so, for by making up an appropriate death rhyme, they can destroy their victims.
These frightening, far-out kids take a black delight in destroying their neighbors, and the Earth people are defenseless against them….
Schlock Value 