I believe that Harper Lee is trying to convey or show that at this time that African-Americans are viewed down upon and that racism is not a problem, which in the time this book was written everyone had grown up being taught that blacks were not equal to Caucasian people. They mention that Calpurnia says, "that's nigger talk" and considering that she is black and she is a motherly figure to these children  and  when she says things like "nigger" it teaches the children that they can say it because they think oh we'll she is black and she says it so it must not be that offensive, so I think the author is trying to show that racism at this time was not seen as a big. Problem because they even purposely exposed children to it.