In "To Kill a Mockingbird", why does Jem think that it's okay for Scout to stay in the courtroom?

In chapter 17, Bob Ewell is on the stand. He is explaining to Mr. Gilmer what happened to his daughter Mayella. He begins to talk about Tom Robinson attacking Mayella. When he states that he saw Tom “ruttin’ on…Mayella,” Reverend Sykes feels that it is inappropriate for Scout to hear. He tells Jem that he "better take Miss Jean Louise home" because he feels that she is too young. Jem tells Reverend Sykes that she "doesn't understand it...she ain't nine yet." Although Scout objects to Jem’s statement and exclaimes that she most certainly does understand, Jem doesn’t want to leave the court and assures Reverend Sykes that Scout will be fine as will he.