Creature of the Night

WorldCat: When Bobby's mother moves the family into a house in the country, a neighbour tells him that a child was once murdered there. Bobby doesn't care. But the longer he spends in the old cottage, the more convinced he becomes that something very strange is going on there. Was there really a murder? And if so, was it the one he has been told about? Wikipedia adds: The cottage they are living in is on a path between two fairy forts. The family is warned by the farmer's mother to put out a bowl of milk every night, but they consider this a mere superstition. Being deprived of the milk, a little old fairy woman comes through the dog flap into the kitchen. Dennis, Bobby's brother, sees and accepts her, but for Bobby it is a baffling and rather frightening mystery.