And I think anyone can do so. True enough, the only kid at my wedding was Bacon, but she's the only one I trusted to behave herself.
I don't see why you can't have just the flower girl (and brother) and no one else there. Why not? They're wedding-party members, not random children. And I certainly would feel no obligation to invite brats. Ever. Even (oh, especially) if it meant that their parents wouldn't come.