Atticus never wants his children to worry. He tries to give them perspective, but he is also optimistic—or tries to be for them.
Atticus tells Scout not to worry when Jem gets upset and destroys all of Mrs. Dubose’s flowers so Atticus sends him to read to her. Scout is concerned because she thinks that Jem is going to get shot.
Atticus pushed my head under his chin. "It's not time to worry yet," he said. "I never thought Jem'd be the one to lose his head over this thought I'd have more trouble with you." (ch 11)
Atticus is telling Scout that there will be harder times once the trial starts. He does not want either of his children to have to fight his battles for him, but he knows that this will affect them. The trial has not started, but the drama has. People know Atticus is defending a black man, and they are not at all happy about it.
In chapter 13, Atticus also tells Scout it is not time to worry yet because Aunt Alexandra has moved in and is making sure that the children behave well. Scout sees a change in him. She assumes it is due to Alexandra’s presence, and it concerns her. Atticus is concerned about the trial, but he does not want his kids to be.
In chapter 22, Atticus also says that it is not time to worry yet. Tom Robinson has been convicted. Atticus is convinced that this is not the end.
"It's not time to worry yet," Atticus reassured him, as we went to the dining room. "We're not through yet. There'll be an appeal, you can count on that. … (ch 22)
Atticus is maintaining home, at least for the kids. He knows that the chances of having a conviction overturned are slim, but he wants to maintain hope for his children’s sake, and to maintain optimism and calm for himself.