Calpurnia arrives at the courthouse with a message for Atticus that his children are missing. This is somewhat comical as the children are both in the courthouse at that moment, avidly watching and listening to Tom Robinson's trial. Calpurnia's entrance also occurs at a moment of high tension when Atticus has just finished summing up, urging the jury to do their duty and acquit Tom Robinson. This has the effect of defusing the tension somewhat at that particular moment.
Atticus was totally unaware up to this point that Jem and Scout have been following the trial so closely. As he has been powerless to stop them, he says they might just as well follow the whole thing through and hear the verdict as well when it is delivered. Having heard all the evidence which to any reasonable mind would appear to be heavily weighted in favour of Tom, Jem is confident that Tom will be acquitted. However, the opposite happens, simply because Tom is black and discriminated against, and Jem is gravely disillusioned. Tom Robinson's trial is his and Scout's first major experience of adult prejudice and injustice, and it is an absolute eye-opener for them. Atticus, it seems, would have preferred them not to attend the trial, but he couldn't have hidden the outcome of it from them in any event.