Let's Kill Uncle

Let's Kill Uncle—also known as Let's Kill Uncle Before Uncle Kills Us—is a 1966 American black comedy psychothriller film produced and directed by William Castle, about a young boy trapped on an island by his uncle, who is planning to kill him. The boy's only friend is a young girl, who tries to help him.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.

Poster © the film's rights holders.