LEO SULLIVAN, who has been director of public relations at the Kennedy Center since before it opened in 1971, is leaving that job to form his own consulting firm. His main client will be the center, for which he will concentrate on special events, the Kennedy Center Honors program, and Stagebill, the program magazine.

Laura Longley, a former editor of The Washington Post Magazine and most recently an editorial consultant, will take over as PR director at the end of October.

Sullivan, who has a three-year contract with the center, said he was not asked to resign, and is looking forward to being free of daily chores in order to devote more energy to the magazine and the special events. "I worked here too long to just abruptly halt my relationship with the Kennedy Center," he said. His firm, Leo Sullivan Associates, will have other clients: He already has his first non-theater assignment for an antique show/benefit. "I feel that I'm on the threshold of a new career," he said.

A former Washington Post reporter, Sullivan handled public relations for the Washington Theatre Club before going to the Kennedy Center. Longley left The Post for Time-Life Books, where she was an editor before leaving to set up her own editorial services firm.

Jessica Tandy and Hume Cronyn, one of the few husband-and-wife acting teams to have endured in the American theater, celebrate their 40th wedding anniversary today. Two of their three children will join them in Baltimore, where they are playing in "Foxfire," to celebrate.

Tandy, 71, and Cronyn, 69, were married in 1942 but did not work together until 1951 in "The Fourposter." "Foxfire," which moves to Broadway in November, is their 11th show together.

In a recent interview, Tandy attributed their marital longevity to mutual respect, good dispositions, and the fact that each had been married unhappily once before. Between them they have roughly 100 years of theatrical experience, and have both been elected to the Theatre Hall of Fame.

Tandy won a Tony as the original Blanche Dubois in "A Streetcar Named Desire," and another for "The Gin Game." Cronyn, who is known as a director as well as an actor, makes his debut as a playwright with "Foxfire," which he coauthored with Susan Cooper.

Although they are devoted to the theater, they have a memorable two-minute cameo in the current film "The World According to Garp," in which they play Garp's very proper grandparents, scandalized by the unorthodox behavior of their daughter Jenny. Tandy does an excellent faint in one scene . . .

The Round House Theatre's designer Richard Young decided that the set for the production of "Cat on a Hot Tin Roof" should consist primarily of wicker, summoning the image not only of southern heat, but an aura of "incompleteness." The furniture was borrowed from antique dealers--but the creation of wicker walls and ceilings presented a challenge.

The solution: concrete reinforcement bars and a material called Ethafoam, which is normally used to mend cracks in swimming pools. Using about a half-mile of the steel rods to form the structure, Young used about a mile of the Ethafoam, a plastic strip material, to weave the wicker. It was all painted a neutral shade, and will be unveiled on opening night, this Thursday.

The New York City Ballet is looking for 24 child dancers, boys and girls between the ages of 9 and 12 and the heights of 4-foot-4 and 4-foot-10. The audition is Oct. 2 at 2:30 at the Opera House . . . The Afro-American Collective Theater has extended the run of "Zooman and the Sign" through this weekend at the Penney YWCA . . . Center Stage in Baltimore opens its 20th season this week with a new play, "Last Looks."