In the short story "Charles," why does Laurie say that all the other children play with Charles even though they are told not to?
We don't have a copy of the full story and don't recall this part of the story.
Shirley Jackson is best known for her shocking short story "The Lottery"; however, one of her most delightful tales is "Charles." It is the story of a little terror in the kindergarten class named Charles. Laurie, a fellow kindergartner, comes home and tells his parents--with great glee--all about how awful Charles has been in class each day. Laurie's parents are afraid their son is picking up bad habits from Charles, so his mother addresses the issue with the kindergarten teacher at the PTA meeting--only to discover that Charles is actually Laurie's alter-ego. The bad boy was Laurie.
The incident to which you refer is found about a third of the way through the story; Laurie is recounting his day to his father:
“Charles yelled so in school they sent a boy in from first grade to tell the teacher she had to make Charles keep quiet, and so Charles had to stay after school. And so all the children stayed to watch him.
“What did he do?” I asked.
“He just sat there,” Laurie said, climbing into his chair at the table. “Hi, Pop, y’old dust mop.”
In short, the answer to your question is that the rest of the kids stayed simply to watch him. Not very exciting, especially since their rebellious hero simply sat there and served out his punishment--this time.