What lesson did Scout help Atticus learn in Chapter 30?

Scout helps Atticus understand that Boo Radley is sensitive and should be treated with care. 

When Atticus finds out that Bob Ewell was killed, he thinks that Jem did it in self-defense when Ewell attacked him and his sister.  Atticus argues with Sherriff Heck Tate about it, because Jem is a minor and he thinks that he will not be severely punished for it.  Heck Tate tells Atticus that Jem had nothing to do with it, and that they also should not involve Boo Radley in it.  The best thing to do is say that Bob Ewell died by accident. 

“I may not be much, Mr. Finch, but I’m still sheriff of Maycomb County and Bob Ewell fell on his knife. Good night, sir.” Mr. Tate stamped off the porch and strode across the front yard. His car door slammed and he drove away. (Ch. 30) 

While Atticus is grappling with this idea, he turns to Scout to see if she understands.  She does.  She knows that Jem had nothing to do with Bob Ewell’s death, and she also knows that they need to protect Boo Radley from any recognition for his heroics. 

Atticus disengaged himself and looked at me. “What do you mean?”

“Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?” (Ch. 30) 

Atticus thanks Boo.  Scout and Heck Tate are both aware that Maycomb would erupt with gossip if anyone knew that Boo Radley saved two children from Bob Ewell.  He is a very quiet and sensitive man, and he would never be able to handle that.  Heck Tate gives the example of a brigade of ladies showing up with angel food cakes.  He tells Atticus that they need to protect quiet, sensitive Boo Radley from that. 

Scout understands.  She has seen Boo Radley, and he has been the backdrop of her childhood.  Boo Radley has always protected her, and this is her chance to protect him.