Where are the children at the resolution of the story?
At the resolution to the story, the children are on the planet Venus. More specifically, they are back in the underground tunnels. More specific than that, they are back in their school building. More specific than that, the children are all standing right outside of the closet that they had locked Margot inside of before going outside to enjoy the sunshine.
They walked slowly down the hall in the sound of cold rain. They turned through the doorway to the room in the sound of the storm and thunder, lightning on their faces, blue and terrible. They walked over to the closet door slowly and stood by it.
On Venus (in the story, not in real life), it rains all of the time, and the sun only comes out once in seven years. Margot had told the children what the sunlight would be like, because she remembers it from her days on Earth. The other children were born on Venus and have never seen the sun. They don't believe Margot, and are angry at her for even talking about something that they have never experienced. To punish Margot, the children lock in her in a closet.