It is written for 9-12 year olds, so about third through sixth grades.
Here is the eNotes overview of Naylor's novel. You can learn more about it by visiting the link below.
"Night Cry is both a suspenseful novel and the story of a thirteen-year-old girl learning to differentiate between what is real and what is imagined. Left alone on a five-acre farm while her father travels selling calendars, Ellen must rely on herself and sort out her fears of the horse that has thrown and killed her brother, of an old woman's eerie predictions, and of a stranger who works on the farm for food. When a celebrity's son is kidnapped, Ellen must overcome her fears and suspicions to rescue the boy. Clues along the way support a satisfying ending to the drama."