IF THE '70s are remembered at all in Hollywood, it may be as the era in which the marketable age for sex objects hit the playground level.

The arrival of Brooke Shields in "Pretty Baby" seemed to set a precedent: that the sexuality of very young girls could be exploited commercially as long as it was carried out "artistically" and "in good taste."

It came in a decade of enormous demand for girl stars in a variety of roles: Linda Blair in "The Exorcist" and Jodie Foster in "Taxi Driver," Tatum O'Neal in "The Bad News Bears" and "Paper Moon," Mariel Hemingway in "Manhattan" and Linda Manz in "Days of Heaven" and "The Wanderers."

And the trend is continuing into the '80s, with the upcoming romantic appearances of: Shields in "Blue Lagoon," a story of shipwrecked adolescents; O'Neal as the teen-age flame of Richard Burton in the love story "Circle of Two" and as the co-star with Kristy McNichol in "Little Darlings," about the loss of virginity at summer camp; and Foster in "Foxes," the tale of four teen-age girls growing up in California -- to name only a few.

Girl stars have become a growth industry. But the trend has been threatened by the legal controversy surrounding a recent "Hollywood Cinderella Story" of a very young actress. And the out come may have lasting effects on the treatment of underage actresses in the entertainment business.

On Nov. 5, 1979, a full-page ad appeared on the inside cover of the Hollywood Reporter, featuring a blonde girl, nude from the waist up, her arms crossed over her chest, and an expression of brooding sensuality on her face. The ad identified her as Tina Payne, and said, "Would You Believe I'm Only 10?"

During the next four days, Tina, a child beauty-pageant queen from Houston, appeared progressively more clothed in the pages of the Reporter and of Variety (which had rejected the first ad because of its "overt sexual content"). The last ad showed her clothed in a long billowy ruffled dress, a child's lunchpail in her hand, "on her way," the copy said, to the family TV series that is her goal.

The cost of the ad campaign, exclusive of makeup, wardrobe, travel and living expenses, was $7,800 -- reportedly paid for by her family. Tina's father, a wealthy Houston industrialist, also leased a home with an expensive Hollywood Hills address while Tina made her "break."

The results were not quite what the Paynes expected. A spate of letters denouncing the campaign (from an organization representing the stage mothers of Hollywood, among others) formed an unprecedented column in The Reporter; similar articles appeared in various magazines. Tina's new-found agent, Mary Grady, debated dropping the fledgling actress because of negative publicity for her agency.

And Tina's mother, Dorothy Payne, as the child's legal guardian, was issued two child labor citations by the Division of Labor Standards Enforcement of the City of Los Angeles: for "taking indecent photographs of a minor, age 10"; and for allowing a minor to work "in ferformance" (as a photographic model) without the required writen permission from the labor commissioner. Dorothy Payne's fines totaled $1,100. She appealed them both at a hearing last month before L.A. Deputy Labor Commissioner Denos Carras.

At the hearings, the commission's attorney charged that, although he did not consider the pictures taken of Tina Payne to be "obscene" or "immoral," it was "indecent" to photograph the breasts of a 10-year-old child in "provocative" or "suggestive" poses, i.e., those normally assumed by an adult. He further charged that, "overall," the pictures taken by photographer Harry Langdon, which included several full-length nudes as well as more innocuous shots, "portrayed Tina Payne as a sex object for the purpose of furthering her career."

Dorothy Payne's attorney argued that the meaning of indecency, as the relevant statute is written, rests largely on the prejudices of the prosecutor. rAs "expert witness" he called in UCLA clinical psychologist Michael Goldstein, who testilfied that the pictures of Tina Payne were not "obscene," and did not display the child's genitalia or show her engaging in sexual activities -- both preconditions, according to Goldstein, for eliciting morbid interest in the "normal" adult. Payne's attorney produced nude pictures of actress-model Brooke Shields, from a book by famed cover photographer Francesco Scavullo, "Scavullo on Beauty." He also cited the movie "Pretty Baby," which starred Shields, as a precedent and standard of comparison.

Both Dorothy Payne and Tom Masters, who masterminded the ad campaign, denied any connection between it and Brooke Shields' success ("A little girl like that running around doing sex scenes -- now that's disgusting," said Masters from his office in the Playboy Building on Sunset Boulevard). But Harry Langdon, the respected industry photographer who shot the ad and who was also appealing a $1,000 fine, said he was given explicit instructions by Dorothy Payne to pattern them after the Scavullo nudes, from which Louis Malle discovered Shields for "Pretty Baby."

Dicisions on both hearings are still pending, and the movie industry is following the story very closely.

Detective Ralph Kenneally, who works in an advisory capacity with the Los Angeles County Commission on Obscenity and Pornography, said last week that the case of Tina Payne (as opposed to child pornography, which shows "real or simulated sexual conduct") does not come under the jurisdiction of California law enforcement. According to a recent Supreme Court decision in the case of In Re Ginnini, he said, the state "is not in the business of slapping legal fig leaves on the variegated forms of the human body."

But, said commission chairman Shirley Lertzman, the group is in process of making recommendations to Plug up holes" in the current obscenity statutes that would "absolutely" be reflected in the entertainment industry. Specifically, these involve "bumping up" the age defining a minor from 15 to 18 years, and eliminating "the selling of obscenity for commercial consideration" as a criterion for prosecution. This means that taking obscene "home pictures" would also be a felony offense: One of the arguments advanced by the Payne attorney was that the full-length nudes of Tina not shown in the ad campaign were for "home use only."

David Raphel, president of marketing at the International Creative Management agency, which controls much of the "packaging" of Hollywood properties, said the decision taken on the Tina Payne case could have a "very important" effect on the industry. Raphel is in charge of marketing Tatum O'Neal's upcoming "Circle of Two," in which the 16-year-old has some "explicit" nude scenes. The film, directed by the Jules Dassin, was shot in Canada, far from the restrictions of the California Labor Board.

"If the law affects 10-, 12- or 13-year-olds it won't have much impact," said Raphel. "If it affects 15-, 16- or 17-year-olds -- the Brooke Shields age -- it could be very damaging. It means there are many scripts around today, possibly many good scripts, which could not be made into films, because distributors need to be careful." He cited the controversy surrounding "Crusing," and said that "the film industry will suffer if this thing is overdone. People today want to see life as it is."

At the same time, the Coogan Law, which originated in the '30s and which governs the disbursing of a child-actor's income, is currently under review by the California state legislature to provide tighter and more encompassing protection. This in turn has turned attention to the general treatment of children in the industry.

"Sexual abuse on the job is a very real problem which every parent of a child who acts will probably face at some point in that child's career," said Dr. Thomas Backer, a clinical psychologist specializing in entertainment career consulting, at a highly charged stage-parents' meeting last week sponsored by the Screen Actors Guild. (Backer said statistics on abuse are being compiled by the California Psychological Association). "When you have an attractive child in a world of adults, someone's going to hit on it. And the exploitation of that child's developing sexual identity is something which is professionally promoted by agents, managers and sometimes the parents themselves."

Meanwhile, Tina Payne did get her break. Just in time for Christmas, she was cast by producer David Gerber in a six-part mini-series about the Old South entitled "Beulah Land," reportedly the most expensive mini-series ever licensed by NBC.

"It's just like selling toothpaste," said Tom Masters. "If you put a new brand of toothpaste out on the shelf without any advertising, it's going to take a helluva long time to move."

"We know Tina's talented," sighed Dorothy Payne. "But there are so many talented children in Hollywood who are just as good as Tina, maybe even better. They only give them 15 seconds in front of the camera. It's just like the pageants. You've got to do something to make the judges remember who you are."

Tina Payne herself is a exceptionally well-disciplined child who wears ribbons in her hair and white eyelet Neiman-Marcus dresses from which her long legs dangle coquettishly and who prefixes an old-fashioned 'Miz" or "Mister" to every adult name. Since the age of 3, she has been a regular on the child-beauty circuit, practicing her pageant skills up to 14 hours a day. Her titles include: "World's Our Little Miss," "Official Yellow Rose of Texas," "Texas Child Crusader," and "World's Most Photogenic Child." o

"You've got to have a gimmick," said Tina after she was asked to demonstrate her talent and had swung into "I Enjoy Being a Girl," replete with miniature bumps and grinds. "I couldn't be No. 55 in the line forever." w ert, they will decorously tap their instruments to applaud, but now in r