In chapter 23, we see that Tom Robinson has been found guilty and is being sent to another prison, while his appeal is going through. Atticus proved that Tom was innocent, and that makes Bob Ewell angry. When Bob threatens Atticus, Atticus just takes it and doesn't do anything in retaliation. Atticus thinks that Bob is doing this, because Atticus made him look like a fool on the witness stand, so he thinks once Bob gets it out of his system, it will all end.
Atticus is well known for the kind of person that doesn't take things to personally. He tries to see the reason why people act the way they are. He tries to see the good in a person. By his reasoning, Bob is just angry at the way Atticus made him look. Atticus is happy to take the brunt of his anger, instead of Bob taking his anger out on anyone else. Jem and Scout are still worried about what Bob might do, but Atticus tries to reassure them.
Atticus doesn't realize that Bob is about to take his anger out in the worst way. Atticus is about to be faced with a parents worst nightmare. Bob Ewell is not the kind of man to let things go. Bob is determined to get his revenge on Atticus one way or the other. He realizes that Atticus is not going to try to fight him, so he goes after the one that matters the most to Atticus. What Bob doesn't realize is that Jem and Scout have a very special person watching over them.