Big Bob and the Winter Holiday Potato by Daniel Manus Pinkwater
When Big Gloria's teacher, Mr. Salami, announces that the class will be creating a play about Santa Claus, Big Gloria objects. Potato Claus, she says, is much better. After all, Potato Claus doesn't sneak into houses at night but comes during the day. He lets kids choose their gifts out of a big book. And he doesn't even care if you've been bad or good!