Bradbury doesn't explicitly tell his readers why Margot doesn't interact with the children. The reader has to make a judgment call on the issue. I think she doesn't interact with the other children because she is newer to Venus and the school than all of the other children. She isn't that terribly new to the school though. The text says that Margot arrived on Venus five years earlier.
And then, of course, the biggest crime of all was that she had come here only five years ago from Earth, and she remembered the sun and the way the sun was and the sky was when she was four in Ohio. And they, they had been on Venus all their lives, and they had been only two years old when last the sun came out and had long since forgotten the color and heat of it and the way it really was.
That should be enough time for her to make friends and find her niche, but she never does. I think a part of her being an outsider is partially her fault. Sure, she is an outsider when she first comes, but I think Margot perpetuates her "differentness" by not attempting to participate in the games with the other children.
And this was because she would play no games with them in the echoing tunnels of the underground city. If they tagged her and ran, she stood blinking after them and did not follow. When the class sang songs about happiness and life and games her lips barely moved.
I don't know why Margot doesn't interact with the other children. The text never says, but the text seems to indicate the Margot is in a perpetual state of depression. Scientifically speaking, that could be because of Seasonal Affective Disorder. It's a depressed mood driven by lack of sunlight. That sounds weird, but it's a legitimate disorder. Margot simply might not be able to muster up the energy to appropriately interact with the other children if she is suffering from severe depression.
The main conflict between Margot and the other children is that Margot definitely remembers what sunlight looks and feels like. The other children do not. That singular fact drives a wedge between Margot and the other children. Some of the children do not believe her, and I think some of the children are flat out jealous of Margot and her memory of the sun.
And they, they had been on Venus all their lives, and they had been only two years old when last the sun came out and had long since forgotten the color and heat of it and the way it really was.
But Margot remembered.
"It’s like a penny," she said once, eyes closed.
"No it’s not!" the children cried.
"It’s like a fire," she said, "in the stove."
"You’re lying, you don’t remember !" cried the children