That the reprehensible Bob Ewell is a bad parent becomes obvious when Burris Ewell appears in Scout's class on the first day of school.
Unlike most children, whose parents ensure that their offspring are clean and neatly dressed before sending them off to school with strict instructions to behave well, Bob Ewell's son displays no indications of parental care or discipline. When his name is called, Burris stands. Scout describes him as "the filthiest human I had ever seen."
Having noticed that Burris has lice, Miss Caroline instructs him on how to rid his hair of these parasites. She also requests that he bathe. Rudely, Burris laughs and says, "I done done my time for this year." When the teacher asks what he means, one of the students explains that the Ewell children only come to school on the first day, and teachers just mark them absent from then on.
"But what about their parents?" asked Miss Caroline in genuine concern.
"Ain't got no mother," was the answer, "and their paw's right contentious."
This student implies that Bob Ewell would be as insolent as his son Burris were he questioned about his boy's behavior. Bob Ewell is obviously also very neglectful of his children's hygiene, and he teaches them no responsibility or respect.
There are quite a few quotes that show that Bob Ewell is a poor parent.
Right in the beginning of the book, Atticus describes Bob Ewell. He is a man who uses all of his relief checks from the government on whiskey. On account of this, his children are hungry and not taken care of. Therefore, the community bends rules to allow Bob Ewell to hunt. Technically this is against the law, but the people know that if they did not bend the rules, Bob's children would suffer. Here is what Atticus says:
“It’s against the law, all right,” said my father, “and it’s certainly bad, but when a man spends his relief checks on green whiskey his children have a way of crying from hunger pains. I don’t know of any landowner around here who begrudges those children any game their father can hit.”
Second, it comes to light in the trial that Bob Ewell struck and beat up his own daughter, Mayella. All the evidence points to this. Here is what Atticus says:
We don’t know, but there is circumstantial evidence to indicate that Mayella Ewell was beaten savagely by someone who led almost exclusively with his left.
Finally, Bob tried to kill Atticus's children.