To no one's surprise, Eunice Kennedy Shriver was again giving more than she was taking. On the White House lawn last Sunday afternoon, she gave Ronald Reagan the chance to become a public backer of Special Olympics, the athletic program for the handicapped and mentally retarded that Shriver created in 1968.

At the White House celebration of the 15th anniversary of the program, it was Reagan who had the plus day. The president hugged the Special Olympians, who had marched before him in the company of Julius Erving and Alberto Salazar. He bantered with the coaches, who were part of the fund- raiser crowd of 1,000 on the South Lawn. Reagan, praising the Special Olympians for "realizing your hopes for a fuller, more productive life," could play a new role: the champion of children.

Beyond the South Lawn, his policy decisions have represented an unprecedented assault on the young. The Children's Defense Fund reports that in real cuts, $9 billion has been taken from programs in the last two years that serve the country's poorest children. With shoot-the-works abandon, the Reagan administration has withdrawn food and health care. It has treated poverty's children as though they were no one's children.

As the newest recruit to the Special Olympics support group, Reagan listened to Eunice Shriver quote President Kennedy on the spirit that has been behind her athletic program and the programs for the poor that her husband, Sargent, and her brother, Edward, are defending today: "We as a nation will be judged by our caring for our least powerful citizens."

The judgment on Special Olympics has an Olympian quality of its own. The program, which began with a track meet in Chicago in 1968 with 1,500 handicapped and retarded athletes, now includes 1 million athletes in 50 countries and in 20,000 towns in 97 percent of America's counties. Next month at Louisiana State University in Baton Rouge, the sixth international Special Olympic Games will be held.

Eunice Shriver currently holds both the indoor and outdoor record in clearing high and low hurdles in the expansion of the Special Olympics. There is hardly a great world athlete, from Pele to Bill Rodgers, who hasn't said yes when Shriver asked for his time. Why shouldn't the monied stars leap out of the clutches of their agents and lawyers when Shriver calls? To help stage a Special Olympic event is to sprint back into the joyousness of sports that gave birth to amateur competition in the first place. Nothing on the American sporting scene is as simple in design or as compassionate in purpose as the Special Olympics.

Should any doubts linger, we are about to witness the commercial unfolding of the other Olympics. In Los Angeles, where the 1984 summer games will be held for 12 days, the race for the gold has already started for some 75 corporations that have been awarded commercial tie-ins. For payment of an average of $4 million, 35 corporations get to call themselves official sponsors.

Perrier is the "official mineral water," M&Ms the official snack food, Anheuser-Busch the official beer maker, General Motors the official truck and McDonald's the official hamburgery. In addition, companies have paid to use Olympic symbols in the advertising and marketing of their products. Moochies Team Outfitting Company has the corner on seat cushions and Ooh La La, Inc., is licensed to sell Olympic jewelry.

Support for the Special Olympics also comes from major companies, but the games bring out their community rather than commercial instincts. There is no Special Olympic truck or beer. None would meet Shriver's standards, for one thing, and the public wouldn't tolerate it for another. The games for the handicapped and retarded appear to be the line beyond which the excesses of the corporate hustle will not be allowed to cross.

The gentleness of a low-budget Special Olympic event is best understood close up. A few years ago, I had the thrill of running a few miles with a Special Olympian who had recently finished the Boston Marathon. Her time was 3:20, which is less than eight minutes a mile. My miles with the young woman were memorable to me because I knew we both understood the thought of Dr. George Sheehan: "Sport is not a test but a therapy."

She ran naturally. She had retained what was learned in Special Olympic meets: get to the finish line on your own time, which will be, for everyone, the best time.