Atticus had originally told Jem and Scout (and Dill) to stay at home on the day of the trial, but after he had come home for lunch, the children decided to take a look for themselves. They managed to attain seats in the balcony of the courtroom amidst Tom's black friends who were attending the trial. The kids remained unseen by Atticus until they were pointed out by B. B. Underwood after Calpurnia arrived to report them missing. The children desperately wanted to see the jury's verdict, since they had already witnessed the entire proceeding (aside from Scout's and Dill's break with Dolphus Raymond).
Calpurnia wanted to "--skin every one of you, alive..." Aunt Alexandra "nearly fainted." Miss Rachel had "run distracted looking for you--" But Atticus realized that the children had already seen the worst of it--the false accusations and charge of rape by Mayella and Tom, and the racist treatment of Tom by the prosecutor, Mr. Gilmer. When Jem begged to "Please let us hear the verdict, please sir," Atticus relented.
"Well, you've heard it all, so you might as well hear the rest."
Atticus later told Alexandra that
"This is their home, sister... We've made it this way for them, they might as well learn to cope with it."
At the beginning of Chapter 21, Atticus finds out that Jem and Scout have been watching the entire trial from the balcony section. Atticus initially tells the children to go home with Calpurnia and not come back. He realizes that Tom Robinson will be wrongly convicted and the news will devastate his children. However, Jem protests and Atticus relents. Atticus reasons that Jem and Scout have already viewed the entire trial and they deserve to hear the verdict. Despite knowing that the verdict will hurt their feelings, Atticus finds it necessary to expose his children to the truth. He wants them to see racial injustice firsthand, knowing that it will end their childhood innocence. In my opinion, Atticus allows Jem and Scout to return to the courtroom because he wants them to see the effects of prejudice and injustice. He wants his children to witness the harsh reality of racial injustice and views Tom Robinson's verdict as a learning experience for Jem and Scout.