Why is Scout afraid to play the Radley game with Jem and Dill?
In chapter 4, Jem comes up with a game where the children reenact Boo Radley's life story. Scout initially refuses to play because she believes that Boo Radley will see them imitating his life's story and kill them in the middle of the night. Scout is still under the impression that Boo is the "malevolent phantom" and believes that he will harm them if he discovers their game. Despite Scout's fear, Jem convinces her to join the game and Scout plays the role of Mrs. Radley. Naturally, Jem plays the role of Boo Radley and Dill plays the role of Mr. Radley. The children title their game "One Man's Family" and reenact various scenes from Boo's life, which are created from rumors and pieces of gossip Jem hears throughout the neighborhood concerning the Radleys. While the children play the game, Scout continually fears that Boo will witness them playing or that her father will find out and punish them. Eventually, the children are forced to stop playing after Atticus questions them about their game.
The fear of Boo Radley is built up throughout the first few chapters of the book as everyone tells some kind of story of a run-in with him or even just being on the wrong end of something he supposedly did. The fact that he never leaves the house only serves to reinforce all the stories.
Though the other kids think that her fear stems mainly from a fear of Boo Radley himself, Scout makes it very clear that she thinks the main problem is that Atticus knows about the game and she is not comfortable with that knowledge. Scout feels that Atticus will not approve of the game, even though she can't quite articulate why.