Atticus agrees to let the children hear the verdict because they've unbeknownst to him already heard the details of the trial in the courtroom. He does send them home, hoping the verdict, and the reactions of the crowd will have died down before they return. Atticus is definitely being honest, but he's also hopeful that he can limit their exposure is possible.
“The jury might be out and back in a minute, we don’t know—” but we could tell Atticus was relenting. “Well, you’ve heard it all, so you might as well hear the rest. Tell you what, you all can come back when you’ve eaten your supper—eat slowly, now, you won’t miss anything important—and if the jury’s still out, you can wait with us. But I expect it’ll be over before you get back.”