Why does O'Connor make the children so obnoxious in "A Good Man is Hard to Find"?

Despite the
fantastic and grotesque nature of events that unfold at the end of the story, O'Connor is at
pains to depict her characters as members of an ordinary, flawed, 1950s All-American family: a
mom, a dad, two kids (boy and girl) and grandma, heading out by car for a family vacation. The
absolute, even comic, typicality of these people highlights how extraordinary the subsequent
events that unfold are. The events are extraordinary--most families will not be murdered by
escaped convicts en route to vacation--but these events are happening to absolutely everyday
people. 

O'Connor is a Catholic writer depicting fallen people in a fallen
world: it is important for the reader to understand that not only the grandmother is flawed.
Imperfect, very ordinary people have to deal with what life throws at them.

Further, the rude way the children treat the grandmother illustrates her lowly status
in the family: the parents obviously allow the children to speak to her rudely. Much of her
desire to cling to her status as a "lady" comes out of her sense of powerlessness:
even her family just endures her. Yet such a woman, difficult and powerless, is about to
experience a profound moment of grace and redemption when she is able for an instant to see her
killer as her son, a child of God.