The Nanny (1965)

Bette Davis is an English nanny whose charge is 10-year-old Joey, just released from a home for disturbed children where he'd spent two years undergoing treatment for drowning his little sister in the bath. Shortly after his arrival home, suspicion arouses again when his mother is poisoned. But Joey continues to insist Nanny is responsible, just as she was with the death of his little sister, with only a neighboring friend believing the young boy.