Smile big and you feelllke "happy days are here again." But smile big. A half-developed smile is not fully guaranteed. Smile until your teeth show. Hi, guaranteed here means that if you do not smile widely, you may not get the same results than if you smile until your teeth show.

As written the meaning of the text is unclear. If it said "the results of a half-developed smile are not guaranteed" or "a half-developed smile is not guaranteed to work" then Carol's interpretation would be correct.