What is Jem thinking when Atticus calls them from the balcony? To Kill a Mockingbird by Harper Lee

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,

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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:

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'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."