France's entry in the Brooke Shields Sweepstakes is a sullen, dark-eyebrowed 14-year- old named Ariel Besse, whose mission, as the heroine of Bertrand Blier's "Beau-Pere," is to seduce her stepfather with such enticing announcements as "I'm a woman with all systems go!" As an erotic tease, both teen-ager and story are in the awkward stage. Neither funny nor sexy, they both drearily pursue the male fantasy about the rapacious nymphet in relentless pursuit of the innocent and helpless middle-aged man. But the man himself, as played by Patrick Dewaere, embodies the passive sex-object idea to such an extent -- he is asleep when the actual seduction finally takes place -- that his sappiness has certain amusement value. In an opening soliloquy, one of several disjointed narrations by assorted characters, this weary man tinkles the piano keys in a cocktail lounge and asks plaintively, "Who knows what's in a piano player's head?" One soon finds out, and the answer is a medley of cocktail-song philosophy about love, loyalty and everything-happens-to-me. The poor slob can't keep a job, a wife or a shave. His best friends are bored by him, his landlord insults him, and when his wife wants to leave him -- "It's hard enough putting makeup on," she says to stop his declaration of love and regret during her toilette -- she makes him push the car for her to get it started. It would seem that nobody loves him -- with one exception. If anything, his resistance to this exception caps his claims to loserdom. Short of beating the child off with a stick when she jumps naked into his bed, he is so thorough in his efforts at innocence that the film cries out for an over-voice beginning, "But Your Honor . . . " BEAU PERE -- At the Key.