Diamonds are forever is a truth in which Ronald Reagan and my children are fellow believers. They mean baseball diamonds.

The other afternoon, when the president wanted to celebrate May as the month of amateur baseball, he invited some 300 youngsters to play ball on the White House lawn. Two of the McCarthy boys, one a first baseman in the mold of Gene McCarthy of Minnesota and the other a manager in the tradition of Joe McCarthy of the old New York Yankees, were among the invitees. Their Friendship Heights team, randomly selected, was one of about 20 that made the White House lawn as well-stocked with talent as it ever was or will be. The atmosphere had the cheeriness of a country fair.

A baseball diamond was set up on the grass before the south portico, which towered as high as an upper deck. The improvised field was bipartisan: it displayed Republican efficiency while looking like a Democrat's public-works project.

An enlightened move by the White House organizers was the inclusion of several girls' teams. Beyond anyone's doubt, including the American Civil Liberties Union's, this put the administration in full compliance with Title IX of the Civil Rights Act, which decrees equal participation in sports regardless of sex. The next time the president is asked at a press conference what he has done for women, he can spin a story about the afternoon that he and baseball defeated sexism on the White House lawn.

A crew of Park Service turf-edgers had placed the bases where the launch pad of Helicopter One is set. What's being launched, said cynics in the White House press corps who were in temporary outfield bleachers 100 yards from home plate, is a Reagan effort to get the baseball vote. Had a lad on the field overheard this, he would have recognized it as a cheap shot from the cheap seats.

The front page of the next morning's Washington Post, a stage the handlers of any president yearn to manage, did, in fact, carry a large photograph of Reagan hurling a baseball to the kids. True as the cynicism probably was, I was taking the afternoon off from it and let it pass by like a wild pitch in the dirt. This was a day, under a high sun, for nothing else but the primacy of baseball. Yogi Berra is said to have noted that baseball is 90 percent mental and 50 percent physical. And on this day, he would be adding, 1,000 percent for children.

The White House understands the leanings of young ballplayers better than I do. I had told my first baseman and his manager that if early exposure to Republican politicians was a risk in life they thought they were ready for, then starting off with Ronald Reagan was a suitable beginning. On the lawn, though, the president's men satisfied the kids' celebrity cravings with higher gods: Jim Palmer and several of his Baltimore Oriole teammates were on hand. So were Stan Musial, Roger Maris and Monte Irvin from yesterday's lineup. The greats, old and current, worked the crowd as if they had been hoarding energy only for this day.

The appearance of Palmer, who is widely known for his beefcake posing in magazines on behalf of the skivvy industry, prompted the press corps to further impieties. Would the athletically trim Reagan, it was wondered, upstage Palmer by coming out in Jockey shorts? Somewhere out there is the underwear vote, too.

Goofy speculation stopped when the president, accompanied by George Bush, once of the Yale nine, walked onto the field. He took to the cheering like Pete Rose rounding third base after a grand slam. Here was Reagan--smiling, looking carefree--enjoying a break from budgets and covert wars. Surrounded by the kids, in colored uniforms as pictorial as stained glass, he was enjoying a flashback boyhood when baseball was all of life.

The flavor of this came out in Reagan's first words to the gathering: "This is more fun than being president. I really do love baseball. I wish we could do this out on the lawn everyday. I wouldn't even complain if a stray ball came through the Oval Office window now and then."

There is a lot of Reagan that is a sandlot baseball coach: a man who would be convincingly earnest about remembering every kid's nickname, a swapper of stories about close plays from years ago, or an amiable battler who gets into the spirit of competition but is not bitter if defeat comes.

The coach in the White House made it known that he's for the children. They, and the rest of us on one afternoon, were for him.