A National Park Service proposal to allow 14- and 15-year-olds to work for private concessionaires in the nation's parks has been strongly condemned by the AFL-CIO as opening the door to possible child-labor abuses.

The Park Service proposed eliminating a longstanding ban on employing children younger than 16 because of a labor shortage at many of the 335 national parks and monuments, according to David E. Gackenbach, director of the concessions division.

More than 500 concessionaires -- including hotels, motels, marinas, service stations, gift shops and tour guides -- hire up to 7,000 summer employes. Many firms complained to the Park Service last year of "no-show" rates of up to 40 percent among teen-agers who were hired but never reported to work, Gackenbach said.

Lifting the ban on younger workers will allow concessionaires to fill those jobs and give youngsters "a new opportunity," he said.

In addition to allowing 14- and 15-year-olds to work, the proposal would lift restrictions for those under 18. Currently, those under 18 may not work more than eight hours a day or six days a week and may not work after 10 p.m.

The only opposition to the proposed rule, published in the Federal Register May 9, came from the AFL-CIO, which raised some of the same objections that the federation has made to the Reagan administration's proposed "subminimum wage" for youth.

The labor federation said the rule would take jobs away from older workers and would expose youngsters to exploitation.

"It is intolerable for the Park Service to back away from its responsibility to protect the health and safety of these youth . . . ," said the letter from AFL-CIO economist John L. Zalusky. "This proposal not only injures young people, it would also result in displacement and unemployment of other persons."

"At the age of 14 to 16, youngsters are not merely smaller workers, they are still maturing physically and mentally . . . Forty hours a week cleaning motel rooms, mowing lawns in the sun, or even doing dishes for a concessionaire is a bit much . . . ," Zalusky said.

He said lifting the ban could also expose youngsters to work in bars and other unsuitable work places, but Gackenbach said the Park Service believes state labor and liquor laws would protect them.