What gave Atticus the idea that something was wrong with the children?

This falls after the trial, in which Atticus has "destroyed his last shred of credibility," "if he had any to begin with." Ewell has already walked up to him in public and spit on him, then told him "he'd get him if it took the rest of his life." 

Atticus blows this off, figuring that Ewell got all his anger out of his system then. The children--Scout, Jem, and Dill--are worried, though. They try to come up with ways to talk Atticus into taking Ewell's threat more seriously, asking him to carry a gun, appeal to his better nature, and Scout even cried and threw a fit. None of this worked. 

The only thing that did was that Atticus noticed how all of them were "dragging around the neighborhood, not eating," and "taking little interest in [their] normal pursuits."