The children have been somewhat traumatized by the constant rain. They desperately want the sun to come out. You could say this made them somewhat bitter and mean, based on the way they treat Margot.
They were all nine years old, and if there had been a day, seven years ago, when the sun came out for an hour and showed its face to the stunned world, they could not recall.
The children desperately want the sun to come out. They are jealous of Margot because she has come more recently from Earth than they have, and she says she remembers it. For this they relentlessly bully her.
Margot does not fit in. In addition to being an outsider and more of an Earthling, she does not relate to the other children. She doesn't try to be one of them.
They edged away from her, they would not look at her. She felt them go away. And this was because she would play no games with them in the echoing tunnels of the underground city.
The sun coming out and the stopping and then restarting of the rain seems to do something to the children. They lock Margot in the closet so she misses the sun's brief appearance, a particularly cruel trick that seems to make even the children realize it's terrible once they have done it.
Rain can be depressing. There is even a name for the feeling people get when the weather makes them depressed. It is called Seasonal Affective Disorder. Imagine how bad this disorder would get if the seasons were seven years long. Even when the sun comes out, it is very brief. People need sunshine.