In Chapter 3, Scout says that "Atticus said the Ewells had been the disgrace of Maycomb for three generations. None of them had done an honest day’s work in his recollection." Atticus had also told her that the Ewell children could miss school if they wanted to and that Bob Ewell could hunt out of season. The reason is that since Bob has no income and since he spends his relief checks on whiskey, the only hope of his children getting fed is if the town allows him to hunt at any time. The people of Maycomb reluctantly tolerate the Ewells' way of life and they have a sympathy for the children.
In Chapter 17, when Atticus is questioning Bob Ewell, he reveals that Bob is left-handed and is therefore more likely to have beaten Mayella. This is one of many instances where Atticus demonstrates what a corrupt man Bob Ewell is. He drinks, abuses his children, and is a useless member of society. From Atticus's questioning of Bob, Mayella, and Tom Robinson, we learn that Mayella is quite lonely and basically has to run the Ewell household and raise the children. It is clear that she had been reaching out to Tom as a friend or companion. Scout adds:
As Tom Robinson gave his testimony, it came to me that Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley, who had not been out of the house in twenty-five years. When Atticus asked had she any friends, she seemed not to know what he meant, then she thought he was making fun of her. (Chapter 19)
In Chapter 23, Atticus explains to Jem why he did not retaliate when Bob Ewell spat in his face. Atticus notes that he was thinking of Mayella. Even though Atticus and most of Maycomb's citizens think the Ewells are "trash," there is sympathy for the children for being born into the Ewell family through no fault of their own. That is why Bob is allowed to hunt in the off season. That is also why Atticus takes Bob's abuse. In discussing it with Jem, he says:
So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there.