Why did Jem not want Scout to tell Atticus about Bob Ewell's comment? Was this a wise thing to ask her to do?This answer will be found in chapter 25.

When Bob Ewell reacts to Tom's death by saying "one down, two more to go," Jem realizes that Ewell's anger has not subsided.  He knows how courageous his father is, but Jem also suspects that Atticus is naive about how deep the feeling for revenge can run.  Jem feels that if Scout tells Atticus about the comment, Atticus might attempt to follow up with the threat and get himself into trouble.

From the standpoint of a 12 year old boy, this is an understandable thing to do.  From the perspective of hindsight, however, it proved foolish.  By telling Atticus, the extent of Ewell's rage might have been realized, and the events of Halloween prevented.