Who Can Kill a Child? (1976)

A couple of English tourists rent a boat to visit the fictitious island of Almanzora, just off the southern Spanish coast. When they arrive, they find the town deserted of adults, there's only children who don't speak but stare at them with eerie smiles. They soon discover that all the children of the island have been posessed by a mysterious force or madness which they can pass from one to another, and which makes them attack and murder their elders, who can't defend themselves because nobody dares to kill a child...