Atticus believes that bad language is a stage that all children go through, that she does not know what she is saying, and that she will grow out of it.
During the Christmas visit with the family, Scout swears often. This is something she is trying out. Even though most adults do not approve of children swearing, Atticus does not seem to care. He thinks that she is just trying to get attention.
Bad language is a stage all children go through, and it dies with time when they learn they're not attracting attention with it. (ch 9)
Uncle Jack tells Scout not to swear, and warns her that he will punish her if she does. When she fights with Francis and he catches her, Uncle Jack spanks her. Jack is surprised when this upsets Scout, because he warned her. She tells him that he doesn’t understand children, and it hurts her that she does not stop to listen to her side of the story.
Atticus's parenting skills come under fire at several points in the book. He has interesting ways of raising children, but his children seem to turn out well. Basically, he does not care what others think. He wants his children to become good people, he does not want to force them into it.
Uncle Jack comes to visit at Christmastime in Chapter 9 of To Kill a Mockingbird. He is unmarried and has never had children. Scout enjoys being around her uncle, but she feels that he does not understand children.
After his arrival, Uncle Jack shows the children photographs of his cat, Rose Alymer. Scout comments on the photos to Uncle Jack:
"She's gettin' fat," [Scout] said.
"I should think so. She eats all the leftover fingers and ears from the hospital."
"Aw, that's a damn story," [Scout] said.
"I beg your pardon?"
Uncle Jack is shocked and taken aback by Scout's cussing. He had not expected his young niece to use such language. The response Atticus has is an unconcerned one:
"Don't pay any attention to her, Jack. She's trying you out. Cal says she's been cussing fluently for a week, now."
Atticus is an understanding parent. He treats his children like adults in many ways. He usually allows them to grow out of unpleasant phases, rather than trying to forcefully suppress them.
Uncle Jack still disapproves of Scout's language, and he takes her aside to have a talk with her. He tells her that he does not want her to use bad language.
Later, Scout asks Uncle Jack what a "whore-lady" is. He does not tell her the truth. He expresses to Atticus again his disapproval of Scout's language. Atticus explains to his brother why he thinks Scout is using cuss words:
"Bad language is a stage all children go through, and it dies with time when they learn they're not attracting attention with it."
Atticus believes that Scout's language choices are merely a common, normal phase. He chooses to ignore the language in hopes that she will quickly realize no one seems to mind.