The kids don't really know what they're getting into when they cross the street and join Atticus and the other men gathered in fron of the jail. It's a mob--a very subtle mob, but a mob nonetheless. A very tense Atticus tries to send them home, but they're not budging. Then Scout sees Mr. Cunningham, Walter's dad and Atticus's client. Scout sees an opportunity to practice what Atticus has taught her--to talk with people about things which are important to them. (Ironically, she sees him do this very thing at the dinner table with Walter Cunningham.) She strikes up a conversation with Walter's dad, talking to him about his entailment, some hickory nuts, and his son.
The entailment is the legal work Atticus did for him; the hickory nuts were partial payment for his legal fees; and his son was a guest in their home. She talked about one thing, then another, then back to the first, then settled on Walter--all with virtually no response from Mr. Cunningham. (I'm confident he had no real clue what to say to this outpouring of a precocious first-grader's conversation.) Finally, dead silence all around. Scout finally asks, "'What's the matter?'" Eventually, Mr. Cunningham bends down and says this:
"'I'll tell him [Walter] you said hey, little lady.'"
And the crisis was over.
Scout had no clue what she was doing, nor did she do anything intentionally. What she managed to do was remind Mr. Cunningham that Atticus is a family man with children; that Atticus is a man who did legal work with no expectation of any payment other than a bag of hickory nuts; that Atticus graciously shared a family meal with Walter; in short, that Atticus is a good man who's simply doing the job he was given. Somehow Scout had figuratively put a real face on Atticus, had personalized him, which diffused the mob mentality and shamed their ringleader, Mr. Cunningham, into leaving.