Atticus leaves the children because the state legislature is called into emergency session. His absence is important for a couple of reasons. The children are left with Calpurnia, and they attend church with her. This event is important because the children get to see how African-Americans live and worship, and learn that everyone is pretty much the same. When everyone returns from church, Aunt Alexandra is waiting for them. While Atticus has all the confidence in the world that Calpurnia can run the household, clearly Aunt Alexandra does not. One might ask whether Alexandra would have come if Calpurnia were white.