When Atticus went out of town for an emergency session of the state legislature, Calpurnia was hesitant to let Scout and Jem go to church on their own.  She thought they needed supervision, so she took them to church with her.

Calpurnia had not forgotten what happened the last time Scout and Jem went to church unsupervised.  Several years before, she "remembered a rainy Sunday when [they] were both fatherless and teacherless" (To Kill a Mockingbird, Chapter 12).  With no teacher to monitor them, the children and their Sunday School classmates had decided to play a game of Shadrach, Meshach, and Abednego.  They tied up a little girl from the class and placed her close to the furnace.  Then the rest of the children clambered upstairs to hear the sermon.   They had forgotten about the poor girl until they heard banging from downstairs.  The girl was discovered, and she was ready to end the game.  

With Atticus out of town and their Sunday School teacher absent, Calpurnia had a decision to make.  This lack of supervision led Calpurnia to make the decision to bring Jem and Scout with her to her own church.