Critics — and even audiences — often wonder what the point of a sequel is. Take “The Matrix Reloaded,” “Speed 2: Cruise Control,” and the recent “The Hangover Part II.” The plot of the last, as funny as the film is, completely mirrors the original. But the question rarely seems to hover around children’s films. Is it that fewer expectations surround the quality of movies aimed at the young? Not when it’s a Disney-Pixar film. (And keep in mind the film opened to midnight showings, too late for most tots.) So “Cars 2” deserves all the skepticism that was focused this summer on the “Hangover” sequel. The animated movie, a follow-up to the 2006 original, is little more than an excuse for the animators to have some fun. Unfortunately, viewers don’t get any until about two-thirds through.