Even for the White House it was quite a mix -- Ronald Reagan, Ted Kennedy, Superman, the Beach Boys and nearly 1,000 others vibrating to a rock concert on the South Lawn last night to help raise money for young mentally retarded athletes in the Special Olympics.

Only Secretary of the Interior James Watt, who started the whole furor over the Beach Boys in April, was missing--and the White House seemed just as glad. As one White House source put it, "If he'd been here, the cameras would only have been on him."

As it was, they were on the Beach Boys more than they were on Superman, actor Christopher Reeve, whose politics have won him few friends at the White House, and the other stars in the new Warner Bros. film, "Superman III." And that wasn't what Warner Bros. had in mind at all when its officials agreed to premiere the film here as part of a day-long Special Olympics benefit package.

"The only objection we had was the way it was originally announced--that the Beach Boys would be at the White House Sunday night for the Special Olympics," said Warner president Terry Semel.

There was no mention of "Superman III," according to Semel, and "it almost sounded like it was a rock concert at the White House."

Which, of course, is exactly what it was. Table companions of the Reagans, in a huge white tent where everybody drank chilled Italian wine and ate supper out of picnic baskets, were Special Olympics founder Eunice Shriver and her husband, Sargent; Warner Bros. chairman Robert Daly and his wife, Nancy; ABC-TV's Frank Gifford and his wife, Astrid; and young Olympian Whitney Berkheimer of Virginia.

Sen. Kennedy (D-Mass.), whose name appeared on the committee guest list but not that of the White House, came with his son, Ted Jr., and Ethel Kennedy was there with several of her children. The Hollywood contingent was scattered around the tent and included actors Robert Vaughn, Richard Pryor and Jackie Cooper. Reeve was there with Gae Exton and their son, Matthew, 3, and Reeve's mother, Barbara Johnson of Princeton, N.J.

"He doesn't feel like iron," said Barbara Cook, deputy press secretary to Nancy Reagan, who positioned Reeve in the lineup with the Reagans.

"I like Superman but I do not like Mr. Reeve's political positions," said another presidential aide.

Reeve, a Democrat, has charged Reagan with "raping the poor people of this country." Last night, in a somewhat mellow mood, he called "Mr. Reegan" a wonderful person and said "the whole experience of being at the White House and being allowed to speak here is a thrill for me. I consider it a real honor."

Nancy Reagan's chief of staff, James Rosebush, said he called the Beach Boys a week ago to ask if there was any possibility at all that they could entertain at the party. "They said they'd love to," he said.

"We were looking forward to seeing them on the Fourth of July," Reagan told his guests, who paid $500, $250 or $70 a ticket. "I'm glad they got here early." At another point, he said, "If you didn't believe that our whole family had been fans of yours for a long time, just look at Nancy."

Her devotion won her a hug from lead singer Mike Love and a song dedication, "California Girls". The crowd seemed to love everything about Love and his friends, including his little dig at the absent Watt, who reportedly was off in Florida giving a speech. "For undesirable elements," said Love, "I think we have a lot of desirable elements here tonight."

Earlier, Reagan blew the whistle that started off a relay race around the White House driveway. It was one of several demonstration events featuring handicapped athletes who will go to the Sixth International Summer Special Olympic Games in Baton Rouge, La., next month.

"If the Special Olympics had waited for government to create, legislate and appropriate a program like this, we'd barely be at the starting line," Reagan said in welcoming the crowd.

Eunice Shriver recalled that her brother, John F. Kennedy, had stood where Reagan stood, "burdened by all the problems that beset the world just as you are burdened now. But when he assessed this country and its role as a world leader he concluded: 'We as a nation will be judged by our caring for our least powerful citizens.' "

"I love that quote by President Kennedy, don't you?" Ethel Kennedy said later.

One young Olympian, Ralph Crossan Jr. of suburban Maryland, surged past security guards, saying, "I want to see the president." He was restrained until Reagan finished his remarks, then was motioned forward by the president, who autographed his cap.