Who is Mr. and Mrs. McFarland from Invisible Monsters and what is their importance?

Mr. and Mrs. McFarland are Shane and Shannon's parents. The McFarland's blame Shannon for Shane's burns after the incident with the hairspray can, believing that Shannon put the can in the trash. Shannon hotly denies this, but later admits she was the one who made several calls to the authorities, causing the child welfare workers to extend their investigation into the family.