What are Scout's and Jem's tones toward Calpurina when asking her about her use of English in her community?

In chapter 12 of To Kill A Mockingbird Scout and Jem accompany Calpurnia to church one Sunday. The entire congregation, like Calpurnia, is African American. Both children notice that Calpurnia speaks differently to members of her church than she does when she is at the Finch residence. Calpurnia explains that she taught her son Zeebo to read using Blackstone's Commentaries, an eighteenth century English law book:

“That’s why you don’t talk like the rest of ‘em,” said Jem.
“The rest of who?”
“Rest of the colored folks. Cal, but you talked like they did in church”

Jem's tone is one of curiosity. He picks up straight away on the fact that Calpurnia speaks differently in church than she does at home. But he still doesn't know the whole story. There is a reason why Calpurnia changes her way of speaking depending on whom she is talking to, as Scout soon finds out:

“Cal,” I asked, “why do you talk n——rtalk to the—to your folks when you know it’s not right?”

Scout's tone, like Jem's, is curious, but she is also being presumptuous in assuming that there is a "right" way of talking. Scout thinks that if you know how to talk in the "proper" way, then there is no excuse for speaking in the kind of dialect used by African Americans.
Scout's tone might also be said to be a little arrogant as she automatically assumes that Calpurnia does not have a life or identity of her own outside the Finch household.
Calpurnia explains that people at church would think she was putting on airs and graces if she talked like she did at home. Yes, she knows better, but sometimes it's best not to let on, otherwise people might think you're trying to make yourself feel superior to them.
This brief episode in the story provides us with another insight into the various hierarchies that exist in Maycomb—different races, different classes, even different ways of speaking that keep people apart.