Atticus has to go to Montgomery for an emergency session of the state legislature. The children are left at home with Calpurnia. Calpurnia realizes that she cannot send Scout and Jem off to church alone. Another time when their father was out of town and their teacher was not there, Scout and Jem had gotten into trouble with their Sunday School class:
Left to its own devices, the class tied Eunice Ann Simpson to a chair and placed her in the furnace room. We forgot her, trooped upstairs to church, and were listening quietly to the sermon when a dreadful banging issued from the radiator pipes, persisting until someone investigated and brought forth Eunice Ann saying she didn't want to play Shadrach any more—Jem Finch said she wouldn't get burnt if she had enough faith, but it was hot down there (To Kill a Mockingbird, Chapter 12).
Concerned that something like this may happen again, Calpurnia decides to bring Scout and Jem to church with her. Usually Atticus makes sure that the Sunday School teacher is present at church each week.
Calpurnia makes sure that Scout and Jem are dressed in their best clothes and clean as can be. She wants the children to make a good impression on the congregants at her church. On Sunday morning, Scout and Jem go with Calpurnia to First Purchase African M.E. Church.