A red-light district - A red light signifies two things basically, something bad or to stop. Well, in this idiom, it does not mean exactly either of the two. But it matches to the former. Well, a red- light district actually means a place where people agree to offer sex with demand for money.
1. Red light districts are the worst places of the world.
2. Kids should not know that this city is so bad and that this is a red light district.
3. I saw you going in the red light district last night, what is the matter Freddy?