Your thesis statement should express the main idea of your essay. The scene you are talking about happens in chapter 12 of the book.
The way you develop a thesis statement about this particular topic is by reading the chapter very carefully—it’s only about 11 pages long. You already know that you are going to focus on what the children learn as a result of going to church with Calpurnia, so pay particular attention to that section. You should take good notes as you read.
If you were my student I would suggest that you focus on how the children are treated by the different church-goers, how Calpurnia’s church is different than their own church, and anything Scout says about her perceptions of the experience. Once you do that, you should be able to come up with a general statement about the fact that it was a learning experience, and then use the body of the essay to give specific examples and discuss their significance.
Don’t be afraid to re-write several times if necessary. Good writers are almost never satisfied with their first efforts.