A stone library lion goes inside for the first time, ransacks the books in the children's room, and, when he's read the last, returns to his pedestal ""satisfied."" The library bears a slight, scaled-down resemblance to the New York Public, and library lion Clemens does have a counterpart. . . who doesn't get to do anything. Only an alert child, probably, will give a thought to that. More problematic is the notion of a stone lion as king-of-the-unseen-library--especially a powder-puff creature like Demarest's Clemens, who seems to have materialized out of the clouds behind him. There's also an elderly-gent librarian hand-stamping books, a rocking horse, and other cozy, storybook touches. Pretty wan stuff, lion-wise or library-wise.