In Ursula K. Le Guin’s Omelas, the festival only works because a child sits alone in a locked room.
Everyone in the city knows. No one can claim ignorance.
That is the point of the story: Omelas is not asking whether cruelty exists. It asks whether comfort built on one visible victim can still feel clean.
Le Guin makes the bargain unbearable on purpose. The room has one child, and the whole city has to walk past it.