The Bench: Who's Billy?

They walked out of the park and towards he parking lot. Then they headed towards a minivan. "Kind of big for only three kids, isn't it?" Biff asked. A four-door sedan would have been enough. "What?" the woman asked. "Mom, you know that Billy isn't here. That's why I asked you to babysit today." "Oh, of course. I just forgot, dear," Biff said. He tried to think. Who was Billy? They all got into the minivan and headed off to their destination, which was ...