THE BLUE LAGOON -- Aspen Hill, Embassy Circle, Loehmann's Plaza, New Carrolton, Springfield Mall and White Flint.

Of the two children-stranded-on-island movies opening this week, "The Blue Lagoon" is both the prettier and the smuttier.

There seems to be a major problem recently with cinema children being cast ashore on photogenic bits of scenery in the sea, where they must practice such local arts and crafts as spearfishing, spying on natives through banana leaves, and building, with their own baby fingers, dwellings that all turn out to look like so many Trader Vic franchises.

In fact, there have come to be certain conventions of the genre, and it's an annoyance that "The Blue Lagoon" keeps violating them. Closeups of exotic but threatening people and animals are interesting -- although the ubiquitous shark is getting to be a bore -- but it's only fair that they then pose some danger, from which the newcomers may be rescued. It's all right to show an advanced degree of civilization being built on the island, but the materials used must either be grown there or be shown as having been rescued from the original wreck.

In "The Blue Lagoon," the scary types are all red herrings, and there's a bottomless trunk of goods taken ashore in the original rowboat -- which, incidentally, is in mint condition a decade later -- so that the hero and heroine are provided with wardrobes that grow along with them. Thus the fascination that ordinarily comes of observing the mechanics of the Robinson Crusoe-Admirable Crighton situation is lost.

But for some 40 minutes, it's a charming and beautiful film. Not quite as beautiful as "The Black Stallion," with which it shares the opening fire-and-shipwreck scene, but beautiful nevertheless. The cinematograper is Nestor Almendros, who did the stunning "Days of Heaven," and his juxtaposition of small children representing fussy Victorian civilization and the blue-green-and-white clearness of the Fiji Islands, is nearly as effective.

But while he keeps dwelling on a fiery sun hitting the ocean's horizon, producer-director Randal Kleiser lights the emotional scene with an equally huge and pervasive smirk.

From the time child-actors Elva Josephson and Glenn Kohan are replaced with the equally pretty but more self-concious adolescents, Brooke Shield and Christopher Atkins, the film turns dirty. This is not a matter of visual sexuality, even though there's almost nothing but nudity from then on -- it's the dialogue, with its silly double entendres, insisting on a relentlessly filthy chuckle to accompany the children's discovery of each other.

The girl's first menstrual period, the boy's masturbation, their growing interest in touching each other -- these are all accompanied by pointedly articulated ignorance, so that the full joke of their not knowing what is going on is crudely emphasized. "What are you doing?" "I saw you playing with it." "Why are you looking at me like that?" "I just keep on having these strange thoughts," and so on.

This is sustained throughout her pregnancy and childbirth to the extent that they are made to not realize what is happening until the baby actually hits the sand. But from the clues given to the audience -- which are, in chronological order, that 1) she finds intercourse painful, 2) she is getting "fat" and 3) she has morning sickness, one can only conclude that the filmmakers know as little of the facts of life as the children.