The Bad Seed (1956) (Region 1 DVD)

Patty McCormack plays Rhoda Penmark, a seemingly sweet, beautiful, smart grade school girl who is in attendance at a school picnic in which a classmate dies. No one suspects that the accident was the result of murder except for Rhoda's apartment building janitor, a simpleton named LeRoy (Henry Jones). But when Rhoda's mother, Christine Penmark (Nancy Kelly), finds out that her real mother is a cold-blooded killer, she begins to suspect her own daughter. Her fears of what Rhoda might do conflict with her protective instincts as a mother, forcing Christina to make terrible decisions--decisions that themselves might be too late.
THE BAD SEED is one of the eeriest films of the 20th century.