"The Blue Lagoon" is a plump sitting duck, waiting to be roasted by sarcastic spectators. But director Randal Kleiser and his associates may enjoy the last laugh at the box office if this oblivious romantic idyll connects with susceptibilities as naive and dumb-founding as their own.

Drawing on a library relic, a 1903 American novel by Henry DeVee Stacpoole (what a handle!) that inspired a British movie with the teenage Jean Simmons in 1948, "The Blue Lagoon" celebrates adolescent sexuality, passion and even paternity in a state of idyllic ignorance picturesquely confused with a state of nature.

Following a shipwreck, first cousins Richard and Emmeline, aged 7, accompanied by the ship's cook, Paddy, become castaways on a tropical island in the South Pacific. When Paddy croaks several years later, the youngsters are left to survive and mature in primitive privacy, free from Victorian constraints and adult supervision.

As nature boy and girl grow up, the movie becomes a picturesque rhapsody to Learning Skills, Playing House, Going Swimming, Enjoying the Scenery and Starting to Feel Sexy in tropical seclusion. Domestic conflicts emerge when puberty begins doing things to the youngsters that they can't comprehend. "I just keep on having these funny thoughts," Emmeline confides. "Tell me, please," Richard replies, to no avail.

Primal experiences are contrasted. Emmeline has her first period, and Richard bloodies himself hunting fresh game. When Richard tries to get amorous after a hearty repast, Emmeline shies way. Annoyed at her reluctance, he stalks out to satisfy himself and gets more annoyed when she turns up to inquire, "What are you doing?" They finally blunder into copulation and procreation. At each step along the way they remain delightfully oblivious. Meanwhile, possible danger lurks across the island in the form of a cannibal tribe.

The juvenile and then teen-age-performers chosen to portray the young castaways contradict the turn-of-the-century context in every word, feature and gesture. As the juvenile Richard and Emmeline, Glenn Kohan and Elva Josephson never suggest a period earlier than the salad days of "The Brady Bunch." As the amorous teen-agers, Christopher Atkins and Brooke Shields evoke modeling sessions and beach-party movies. It's as if Gidget and Moondoggie had been teleported from Malibu to the South Seas.

The inability of these kids to draw obvious inferences from the glorious natural environment that surrounds them and the juices supposedly rising within them turns into a running gag. The only time either character makes a mental connection is when Richard stumbles upon Paddy's bleached skeleton, gets and a puzzled look and then begins feeling for the bones hidden beneath his own bronzed skin.

Richard confesses that "There's so many things I don't understand . . . Why do you hear the sea in shells? Why are all these funny hairs growing on me?" Everyone within earshot must have been hypnotized by the breakers when Shields was allowed to refer to her budding breasts as "buppies," a new euphemism that suggests something between a guppy and Rowan & Martin's vaguely lewd bippy.

Kleiser resents the critical pans that accused the movie of purveying "kiddie porn," a wild exaggeration derived from fleeting moments of mostly submerged nudity, with the camera peering up from the depths at youthful silhouettes gliding upon the shimmery surface. The average department store catalog is really more provocative than "The Blue Lagoon."

Richard and Emmeline are bound to look silly when they never catch on to anything. Moreover, the camera is always intruding and then shying away from explicit eroticism.

Kleiser and cinematographer Nestor Almendros no doubt had a marvelous time contemplating the flora, fauna, seascapes and sunsets of the Fiji Islands for three months, and it's a pleasure to share some of the scenic footage they returned with. Nevertheless, they never come close to the achievement of Carroll Ballard and Caleb Deschanel in "The Black Stallion," in which the picturesque was transcended and began to illuminate truly elemental longings.

Kleiser has left many a red herring to decompose in incrimination sight. The biggest whopper is that menancing tribe of cannibals, who never become the promised climactic peril. The only sequence in which Shields recalls the precocious, compelling eroticism of "Pretty Baby" is a brief nocturnal visit to the cannibals' sacrificial altar in the jungle. From her ambiguous smile, one might jump to the conclusion that she hankered to become the next sacrifice.

Shields was never allowed to appear in the nude in "The Blue Lagoon." Emmeline's nude scenes were doubled by Kathy Troutt, a lithe Australian Stuntwoman of 32. It seems appropriate that most of the prurient interest this film may arouse will be based on a misconception.