Jem and Scout learn that just as some white citizens show prejudice in Maycomb, so do some black citizens.
As Jem and Scout approach the church with Calpurnia, they are at first greeted respectfully. But then a black woman named Lulu confronts Cal, telling her, “You ain’t got no business bringin’ white chillun here—they got their church, we got our’n. It is our church, ain’t it, Miss Cal?”
At this point, Jem wants to leave, but then other members of the church greet them warmly, and they stay for the service.
They also learn that church is conducted differently in the black community. They sing without hymnbooks. Reverend Sykes is very direct with his congregation, insisting that they increase their offering in order to help Tom Robinson’s wife get by. He also calls out church members by name and mentions their transgressions.
This is a great question. Scout and Jem learn a few valuable lessons, even if they will understand these lessons later in life.
First, they learn that the black community is poor and have little compared to them. For instance, when they are at church, Reverend Sykes is trying to raise money to help Tom Robinson's family. It is not a huge amount of money, but there is a need.
Second, they also learn that many blacks cannot read. For example, they realize the blacks do not have hymn books. When they ask why, Calpurnia says that many of them cannot read. So, the song leader sings a line, and other follow.
Third, they also learn about the trial. This is when Scout asks about rape. Calpurnia has the good sense to deflect this question to Atticus.
Finally, when the children go to the church there is a black lady (Lula) that is enraged that Calpurnia brought white children to a black church. Hence, they learn that racism goes both ways.