In Chapter 21 of To Kill a Mockingbird, Harper Lee portrays the contrast between the idealistic thinking of the boy Jem and the realistic reasoning of his father. First of all, after Mr. Underwood tells Atticus,
They're right up yonder in the colored balcony--been there since precisely one-eighteen P.M.
Atticus is upset that the children have heard all the testimony by being in the courtroom, so he calls to his son, "Jem, come down from there." As the children approach, Calpurnia looks "peeved," Atticus "exhausted."
Then, the idealistic Jem, "jumping in excitement," does not notice the weariness of his father, or the exasperation of Calpurnia as he exclaims, "We've won, haven't we?" Nor does he understand Atticus's curtness:
'I've no idea,' said Atticus shortly. 'You've been here all afternoon? Go home with Capurnia and get your supper--and stay home.'
But, Atticus relents and allows the children to return after eating supper "slowly."
Jem and Scout both think they are in big trouble when Atticus looks up and sees them in the balcony during the Tom Robinson trial in To Kill a Mockingbird. The children had been told to stay at home, but their curiosity got the best of them, and they ventured to the courthouse. They saw there were no seats on the ground floor of the courtroom, but Reverend Sykes invited them to sit in the Negro section in the balcony. When they see Calpurnia approach Judge Taylor with a note for Atticus, they expect a lecture from Atticus. But after a lunch back home, Atticus allows them to return for the remainder of the trial.
Jem also was thinking about something else that would later prove to be false hope.
Jem was jumping in excitement. "We've won, haven't we?"
I think his immediate response is that he's busted. He is being caught. Atticus had told the children that he didn't want to see them anywhere near the courthouse today. He expressly forbid them from going and they did exactly what they had been told not to do.
This is significant because Jem so looks up to who Atticus is. I also get the sense when the kids first talk to Atticus that Jem is so excited about what is going on with the trial that Jem thinks Atticus will want to celebrate his coming victory with the kids. It is a bittersweet moment for Jem.
You can find the answer to this in Chapter 21. It is right at the beginning of the chapter.
At the end of Chapter 20, Calpurnia comes into the court room. She has a note telling Atticus that his kids are missing. Atticus is worried, but the judge tells him that the kids have been up in the "colored balcony" for quite a while. Atticus calls for them to come down.
When Jem gets down, he is all pumped up because he feels that "we" (meaning Atticus and Tom Robinson) have won the case and Robinson will be acquitted.