"A Little Romance," opening today at area theaters, is a precious trifle about two bright teen-agers-a movie-crazy French boy who lives in Paris with his cabbie father and a studious American girl who lives in Paris with her flighty mother and wealthy step-father. The kids, a precocious, well-read pair, conceive a sentimental hankering to visit Venice so they can kiss at an appointed hour beneath the Bridge of Sighs. Wait: It gets worse.

Thanks to the heavy synthetic hand of director George Roy Hill, the potentially charming aspects of th kids' infatuation curdle into syrupy gruel.

Poor Venice itself looks ready for permanent retirement after serving as the defenseless scenic love object in Hill's juvenile idyll. So much adoring goo adheres to the setting that you don't care if you ever set eyes on those canals and piazzas again.

The movie also exploits Laurence Olivier for mawkish effects better left unwitnessed. Oliver's mind-boggling thank-you at the Oscars now appears to be an extension of his performance in "A Little Romance" as a weary, garrulous old imposter who helps the kids realize their dream date.

It's humiliating to watch Olivier pretend to embody this dear old fraud. Hill has so little shame or judgment that he even tosses in a scene of Olivier being roughed up by Venetian cops, who supposedly think he's kidnaped the runaway youngster.

True to his vow, that battered codger keeps quiet about the kids' whereabouts until he hears the tolling of the campanile bells and knows that the magic kissing hour has passed. Harking to the bongs, Olivier looks hilariously similar to George C. Scott contemplating the Gates of Paradise in the first part of "Movie, Movie." The trouble is, Olivier isn't supposed to look funny.

Hill's sentimental abuses are anticipated in his brusque attempts to evoke the New Wave early in the film. The photography is supposed to recall Truffaut, but somehow Hill doesn't achieve the same fleet, airy style of composition. "A Little Romance" looks rushed, cramped, overexplicit.

Since the New Wave has been dead for some time, it makes little difference that Hill's synthetic homage buries it even deeper. The misuse of the young leads. Diane Lane and Thelonious Bernard, is unfortunate, because they do appear bright and appealing at first and might have carried a story about adolescent friendship.

Hill seems to have a knack for selecting attractive young actresses who can really embody the idea of intelligent, well-bred teen-age girls. Diane Lane is even more satisfying on this score than Tippy Walker was in Hill's "The World of Henry Orient" back in the early '60s. Bernard is a likable, puckish kid; he looks as if he might be a remote, juvenile, French relative of John Belushi's.

Aggravating a bad habit from "The World of Henry Orient," Hill and writer Allan Burns give the precocious young heroine a negligent bird-brain of a mom. Sally Kellerman does funnier things with this insulting role than it deserves, and David Dukes is also very amusing as her equally dim-witted new amour.