In Chapter Four of To Kill a Mockingbird, Jem decides that the group should play a new game: acting out the life of the reclusive Boo Radley. In this little drama, Scout plans Mrs. Radley and is responsible for sweeping the porch, while Dill plays Mr. Radley and must walk up and down the sidewalk. Jem plays Boo himself and hides under the porch steps, howling and shrieking. The kids rehearse this over and over, adding a plot and dialogue. Atticus eventually catches them in the middle of their game and responds to their secretive behavior by telling them that he certainly hopes the game has nothing to do with the Radley Family. 

Thus, Scout cites two reasons for wanting to quit the game. First, on the day that she had gone rolling down the sidewalk in a tire and landed squarely in the Radley's front yard, she was certain that she had heard someone laughing from inside the house through all the chaos. Second, Atticus' arrival home and his discovery of their game makes her realize that they might be doing something wrong; Scout knows that Atticus would not be pleased if he discovered that they had lied to him about the theme of the game. This, however, is not a convincing argument for Jem, who decides that they can still play the game as long as they change the characters' names.

Why does Scout want to stop playing the game?  

The "Boo Radley" game begins in chapter 4 of To Kill a Mockingbird. The game involves the children playing various roles, with the part of Boo Radley belonging to Jem. Their drama is "woven from bits and scraps of gossip and neighborhood legend."

Scout wants to quit the game for two reasons. One day, Atticus happens to see the children playing the game. Atticus asks, "Does this have anything to do with the Radleys?" Jem denies that it does. This is the first reason Scout wants to quit the game. She doesn't want to be in trouble with Atticus.

The second reason Scout wants to quit the game involves a previous incident. Earlier, when Scout, Jem, and Dill are playing outside, Jem rolls Scout in a tire. She lands in the Radley's front yard after bumping into the steps of their porch. She doesn't share this information with the others, but when she falls out of the tire, she believes that "Someone was inside the house laughing." This is her second reason she thinks it would be best to stop playing the "Boo Radley" game. She is fairly certain that Boo must have seen them.