If Malcolm Forbes didn't exist, it would be desirable to invent him.

Forbes is obscenely rich and gloriously un-grownup, and he shares his toys. A round 12,000 of them are set out for us to envy and enjoy at the National Geographic Society's headquarters. Toy soldiers, toy tanks, toy planes, toy potentates -- toys, toys, toys. Toys of the sort and number we used to dream about, toys of sorts and in numbers we didn't know how to dream about.

Take the tanks, for instance: In a display representing the German army in North Africa, they swarm across the sand in squadrons, divisions and corps, endlessly wheeling masses of armor and support troops.

"This is just a sampling of this part of the collection, actually," said Peter Johnson, co-curator of the Forbes Magazine Museum of Military Miniatures in Tangier. "We have many more Afrika Korps pieces back in Morocco."

How many more?

"Well, we have a good deal more materiel than Rommel had available at any one time in the Western Desert."

Explorers Hall curator Peter Purpura has been putting on dazzling displays at the Geographic for years, but he's outdone himself this time. On the way into the exhibit, we get to peek into the bedroom window of the kind of rich kid we all used to know and hate. He had, in the first place, a room of his own, and it was full of toys he'd show you but wouldn't let you play with. This kid who lived up the block from me -- his name was Bryce -- boy was he a rat. He. . .

But about the Geographic exhibit. Well, this kid is sick and he has to stay home in bed. In my day it would have been scarlet fever and you'd feel bad about being glad he was sick, but you'd wonder, nevertheless, what was to become of all those toys if the rich kid should, you know. . .

Anyway, this kid at the Geographic, he's lying in bed and he has all these toy soldiers spread out, more toy soldiers than I ever had altogether, including the ones I lost or stepped on or the dog chewed up.

The twist is, this kid that Purpura has created doesn't have a head. What you do is stick your head into the display and there you see yourself in the mirror opposite the bed -- you're the rich kid. At last.

The illusion lasts just for a moment, then you remember that Malcolm Forbes is the rich kid and these are his toys, and all the wishing in the world won't help you get your hands on them.

But it's hard to hate Forbes, because it ain't easy being a sexagenarian rich kid. Forbes is a professional rich kid, with hot-air balloons and boats and planes and whatever, more whatever than he and his sons can handle, so that he has to hire people to keep track of his toys, and play with them. Forbes' fun comes from showing them off; the very rich are less diffident than you and I.

Anyway, here are a dozen thousand of his toy soldiers, in wonderful array. The centerpiece of the exhibit is a French Foreign Legion outpost under assault by Arabs. Unlike the other displays, which were created for the exhibit, this one was transported entire from Tangier, where it's a popular item in Forbes' museum in the grand Palais Mendoub. Why is the museum in a palace in Tangier? Because he wanted it there; stop asking silly questions. The fort and figures are the same, but there was a sea change: At the Geographic, the French are defending the fort against hordes of Arabs; in Tangier, it was set up so that the Arabs were holding the fort against the French. With the Arabs winning. When in Rome. . .

The exhibit's range is suggested by some of the faces seen in little: stout Cortes, Kaiser Wilhelm II, Caesar and Cleopatra, Achilles, Emperor Menelik of Ethiopia, King Boris of Bulgaria, Napoleon, King George and Queen Mary, Edward III, the Black Prince, Rommel, Sherlock Holmes and Dr. Watson, Santa Claus, Alexander the Great, George VI, Elizabeth II, Henry VIII, Sir Walter Raleigh, Odysseus, Zeus, Nansen (of the North), Hitler and Mussolini (giving the fascist salute), von Hindenburg (refusing to give the fascist salute), George Washington and Uncle Sam.

Peter and Anne Johnson, co-curators of the Forbes Museum, are a scholarly yet puckish couple who demonstrate why there'll always be an England: They are dead- serious about military (and other) miniatures, and can tell you anything you want to know about the history of the craft or the historical ramifications of the dioramas; yet neither has lost sight of the fact that these are toys. Describing some one-of-a-kind modern "solids" -- lead castings of mounted medieval knights that go for $500 each -- Anne Johnson shrugs delicately, suggesting it's all a bit much, and turns to a discussion of the British genius for costume. "We went at it right from the beginning, you know; before we had cloth we painted ourselves blue."

Geographic people were horrified the other morning to find Peter Johnson scorching and melting some tanks with his cigarette lighter, an unthinkable curatorial crime. "Makes it look more real, don't you think?" he said.

The collection -- there are 68,000 more pieces back in Tangier -- represents 200 years of toy-soldiering in Europe and the U.S., from early German "flats" to the centrifuge- cast English "hollows" that brought full-round figures within the price range of the masses. There are no plastic soldiers, which Forbes despises, except for one squad of Russian ones. They're awful; they make one's heart ache for the little Russians who have to play with such things. That's probably the reaction we're supposed to have: Forbes describes his magazine as "a capitalist tool."

Many of the toys tell more than their makers intended about the countries they come from. When World War II broke out, the Allies immediately banned their manufacture because lead and tin were in desperately short supply; patriotic children turned in tons of tin soldiers to make bullets for real soldiers. But in Nazi Germany, by Hitler's order, toy-soldier production went on. Der Fuehrer, building toward the Thousand-Year Reich, made sure that the Nazis of the future had plenty of realistic stormtroopers (and scruffy-looking Allied prisoners) to play with.

The display's captions are informative, even solemn. This is, after all, Explorers Hall, normally given over to the fruits of the labors of heroes of science and discovery. The Geographic seems a trifle embarrassed to be making such a to-do over Forbes' frivolity. It need not be, for the exhibition embodies that great and valuable truth about the human condition uttered by Dorothy Parker:

"The only difference between men and boys is the cost and number of their toys." ON PARADE: A PAGEANT OF TOY SOLDIERS -- Through Easter at the National Geographic Society, 17th and M streets NW. Open Saturdays and holidays 9 to 5, Sundays 10 to 5, weekdays 9 to 6. CAPTION: Pictures 1 and 2, A warrior enters the serried ranks of a National Geographic display of Malcolm Forbes's toy soldiers; Persian camel archers take aim at the enemy. By Joseph H. Bailey Copyright (c) 1982, NGS