What is Calpurnia doing in the courtroom inHarper Lee's To Kill a Mockingbird?

Calpurnia comes to the courtroom in order to deliver a message to Atticus Finch.

In Chapter 21, Calpurnia comes to the courtroom in the afternoon during Tom Robinson trial and stands tentatively at the railing. When Judge Taylor sees her, he says, "It's Calpurnia, isn't it?" She, then, asks to give Mr. Finch a note, adding that it has nothing to do with the trial.
The contents of this note reveal her purpose: Alexandra has written that the children are missing.

Since Atticus has given the children strict instructions at breakfast that he does not want them downtown this day, Aunt Alexandra does not think that they have disobeyed. But, in the narrative Scout tells readers, "after dinner, we stopped by for Dill and went to town." They just can not resist attending, especially after all the talk about the trial and Atticus's own earlier remarks to his brother Jack that he could not face his children were he not to accept Judge Taylor's assignment.

Having read the note, Atticus speaks to the keenly alert Judge Taylor, who tells him that the children are in the balcony, and they have been there since 1:18 p.m. Atticus turns and orders Jem to come down the stairs. After descending, Jem begs Atticus to let them stay and hear the verdict; Atticus relents, "Well, you've heard it all, so you might as well hear the rest." Still, Atticus sends them home for supper with permission to return afterwards.