In chapter 21 what does Jem expect the verdict to be? Does Atticus think the same?

Jem is idealistic: his idea of justice is based on what should be, not the reality of the community in which they live. When he sees Atticus, he asks, "We've won, haven't we?" Reverent Sykes takes a more prudent point of view, saying “Now don’t you be so confident, Mr. Jem, I ain’t ever seen any jury decide in favor of a colored man over a white man.” He attempts to soften the blow before Jem's optimism is destroyed by the racism of the town.

Atticus knows the true nature of the men sitting on the jury. He tells Jem that the jury most likely won't be out long, & when Jem asks if that's because they'll acquit Tom quickly, "Atticus opened his mouth to answer, but shut it and left [them]." Atticus knows he will lose, but he also knows that he has done the right thing in defending Tom, & not only that, but he's done his best as well. He didn't just take the case & go through the motions; he fought as well as he could, using his moral strength to guide him. Even though he doesn't want to disappoint Jem, and ruin his idealism, he knows it's important for his children to truly understand. So he allows them to watch the verdict being read. Even though it may be painful (and indeed, Jem cries with anger), Atticus sees this moment as essential in his childrens' maturation.