Why do Scout and Jem go with Calpurnia to her church in Lee's To Kill a Mockingbird?

Atticus was not home, so Calpurnia brought Scout and Jem to church with her because she didn't want them going by themselves.

Calpurnia was suspicious of the children being left to their own devices at church, because apparently one time they went to Sunday School and the teacher wasn’t there and there was trouble.  Scout and Jem had been expecting to go to church on their own, since Calpurnia went to a different church, but...

Atticus was not home, so Calpurnia brought Scout and Jem to church with her because she didn't want them going by themselves.

Calpurnia was suspicious of the children being left to their own devices at church, because apparently one time they went to Sunday School and the teacher wasn’t there and there was trouble.  Scout and Jem had been expecting to go to church on their own, since Calpurnia went to a different church, but she decided not to risk it.

“Besides, Cal, this isn’t the first time Atticus has left us,” I protested.

“Yeah, but he makes certain your teacher’s gonna be there. I didn’t hear him say this time—reckon he forgot it.” Calpurnia scratched her head. Suddenly she smiled. “How’d you and Mister Jem like to come to church with me tomorrow?” (Ch. 12)

Scout and Jem have never been to the African American church, which is called First Purchase because the community gathered its own money to buy it.  It was “paid for from the first earnings of freed slaves.”  It also has no white members.  This is why Lula is surprised to see Scout and Jem, and makes a big deal out of it.

“I wants to know why you bringin‘ white chillun to nigger church.”

“They’s my comp’ny,” said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.

“Yeah, an‘ I reckon you’s comp’ny at the Finch house durin’ the week.” (Ch. 12)

Calpurnia made a big fuss about bringing Scout and Jem to church with her.  She starched Scout’s skirt so much that it flew up when she sat down, and she scrubbed both of them to death.  She wanted to make sure that the children did not embarrass her at church, and then Lula questioned their right to be there.

Then Cal’s son Zeebo intervened, telling them that Lula was just a trouble-maker.  Scout and Jem had a very eye-opening experience at the church.  For one thing, they learned that most of the members could not read.  They did not have a black school.  Zeebo did not go to school either, but Cal learned to read and she taught her son.