The Moorchild, by Eloise McGraw, 1996

Old Bess could tell that the baby in the truckle bed was different than before -- its eyes changed color, and it screamed night and day, especially when the iron-wearing blacksmith came indoors! But Yanno and Anwara, the baby's parents, wouldn't hear what Old Bess had to say: that the baby was a changeling, one of the Moorfolk, swapped for their own infant. So Saaski grew and tried to do as a human child would, but that didn't stop the villagers from calling her freaky-odd, and blaming their troubles upon her. Engrossing middle grade fantasy, complex, psychological, atmospheric. Most strongly recommended.