In this chapter of the novel, there are a couple of instances of strange behavior from Atticus, which make the children suspect (correctly) that something is wrong. Near the beginning of the chapter, a group of men arrive on the Finches' doorstep, and Scout overhears an altercation about Tom Robinson. Later in the chapter, Atticus, carrying a light bulb and an extension cord, tells the children he's going out and proceeds to drive his car to the jail. Scout and Jem follow to find him reading outside the jail by the light of his lightbulb. As the children watch, a group of men arrive and begin to issue threats. At this juncture, Scout, afraid for her father, runs out to protect Atticus, who is rightly afraid; he tells Jem to take her, along with Dill, back home. When Jem protests this, he's grabbed by one of the threatening gang, who Scout then kicks.
It's at this juncture that things seem very tense and as if they could go horribly wrong, but Scout then does something unexpected—she begins to address Mr. Cunningham, the only person in the crowd she recognizes. The very power of her childish prattling is in the normality of it—she asks him ordinary questions about Walter and about his entailment. While he does not answer for some time, eventually he gives in, tells her he will say hi to Walter for her, and tells the others that it's time to leave. Scout doesn't understand how she has caused the violence to dissipate, but effectively, she has shamed Mr. Cunningham into leaving. Her childish innocence makes Tom's murder impossible. He is unable to continue behaving this way in front of a girl who clearly believes him to be an upstanding and ordinary adult. As a result, the altercation is, for the moment, averted.