Jem and Scout have a right to know what his father is up against and what he is representing. The kids see Aunt Alexandra's "missionary teas" all of the time, and those women are just a bunch of bigoted gossipers. The kids are surrounded by racist remarks, even by their own neighbors such as Miss Stephanie and Mr. Avery with their comments on where the kids sat in the courtroom.
Atticus and his family are up against a lot of hatred. He wants his kids to know what is right, even if it is a bit of a slap in the face. He doesn't want them directly involved, but he wants them to question their neighbors and question their actions. That is the only way they will truly see and learn right from wrong. So when he says that they have a right to see for themselves, it's because he belives it is part of their education, even if it isn't pretty to watch.
In Harper Lee's novel To Kill a Mockingbird, Atticus Finch is a lawyer who ideally would like to shield his young children from the ugliness of the world in which they live, but he knows that, ultimately, he cannot succeed. Jem, in particular, presents a challenge to his father because he is older and more mature than Scout, who is only six years old when the story begins. Scout is intelligent and inquisitive but is also possessed of the immaturity befitting someone of her age. While the children age during the course of the novel, Jem remains the more mature for the obvious reason of age. He is also, however, the recipient of countless exchanges with his educated, wise, and patient father.
Atticus did not want his children in court because he knew they would be exposed to the lies, deceit, and racism endemic to the society in which his family resided and because of the abuse that would invariably be heaped upon his children's father by virtue of his willingness to defend Tom Robinson. He ultimately relents, however, because the children have surreptitiously observed the court proceedings from the upper balcony where the African American citizens of Maycomb are seated. It is late in the trial when Atticus is informed of the presence of Jem and Scout, and he decides that they might as well remain in the courtroom to witness the outcome of the proceedings they have already observed. Atticus knows that Jem will remain both curious and concerned about the trial and that the young boy's initial optimism will prove ill-founded. As Chapter 26 comes to a close, Scout describes the father-son relationship in the context of the travesty of justice the children recently witnessed:
"Atticus said that Jem was trying hard to forget all the prejudice and injustice he saw at Tom’s trial. After enough time passed, Jem would be able to make sense of it all and sort things out, but right now he was very upset."
Throughout To Kill a Mockingbird, Atticus has patiently counseled his young son on the realities of life. To try to shield Jem from the truth surrounding the trial of Tom Robinson would represent too grave a departure from that history.
Atticus doesn't want them to loose their innocence and see the blacks treated in unjustice and unfairly