Please explain the irony of Scout's statement about the "pink penitentary" in Chapter 14 of "To Kill a Mockingbird." “I felt the starched walls...

The lines before your quotation say,

"You've got to do something about her," Aunty was saying. "You've let things go on too long, Atticus, too long."
"I don't see any harm in letting her go out there. Cal'd look after her there as well as she does here."
Who was the "her" they were talking about?

The irony is that Scout assumes Atticus and Aunt Alexandra are talking about her. She is so frightened that things will change she wants to run away. However, the subject of the conversation is Calpurnia. Aunt Alexandra wants Atticus to fire Calpurnia but Atticus refuses.