Something joyous happened on the downtown-bound No. 42 Metrobus yesterday morning. It started out as just another 60 cents into the coin machine, another lurching walk down the aisle in search of a seat among strangers, another maddeningly slow ride along Columbia Road NW, headed for Connecticut Avenue. The passengers were sullen and silent. Through the closed windows of the bus they saw the splendid morning, but it seemed somehow inaccessible.

Then the children got on.

Giggling and chattering, they scrambled aboard at the 18th Street stop in front of the Standard Drugs next to Al's Sub-Preme. At first they were lost among the long line of passengers, and from the rear of the bus all that was first known of their presence was the glad sound of them. Moments later they were all back there--17 four-year-old boys and girls, a handsome brown-eyed, black-haired group led by three women, all carrying with them a slice of the morning's splendor.

The seats were all taken, which turned out to be a good thing because this allowed the regular passengers, many of whom never talk to anyone on the bus, to befriend the children, if only for a few blocks. They smiled and reached out their arms and took the children onto their laps as if they were their own. The young passengers did not seem like strangers, partly because each child's name was printed with black magic marker on a big blue piece of paper that dangled from their necks from orange yarn.

The adults called out the names, which were, for some aboard the bus, foreign-sounding and difficult to pronounce: "Karen Torrico, Billy Escoto, Walter Escoto, Tracy Alfonso, Manuel Orosco, Jennifer Solis, Henry Leonzo, Lorena Chavez, Leslie Funes, Javier Flores, Paola Lozada, Enrique Puentes, David Arias, Jennifer Dubon, Omar Hernandez, Rosa Marie Lopez, Veronica Martinez."

A young man who said he came here eight months ago from Ethiopia and works in a Maryland furniture store shared his seat with Karen and Tracy, who happily squeezed in next to him. He caught their delight with the morning. "Washington is like Rome," he said as the bus rounded Dupont Circle, where the water danced in the stone fountain. "I've been to Rome."

The people on the bus, trying to charm the children as they seldom tried to charm each other in the morning, wanted to know who they were and where they were going. Marta Palacios, their teacher, gave some facts. They are the four-year-olds from the Spanish Education Development Center, a bilingual multicultural day-care center at 1840 Kalorama Rd.

They are children of Latin American immigrants, living in the neighborhoods of Adams-Morgan and Mount Pleasant. Their parents work as housekeepers, cooks, waiters, construction workers and secretaries and are grateful for the Department of Health and Human Services-funded center, which charges them according to their incomes to keep their children from 7 a.m. to 6 p.m.

On this morning, at 10:25, they had just come from their Afro-Latin dance class in a studio on 18th Street and were on their way to the Museum of Natural History, where they would visit the exhibit called "South America: Continent and Culture." They ride the bus wherever they go in the city, but for them the trip was not routine. It was an event of great excitement. The sudden stops and turns that so irritate the regular passengers thrilled them. The bus ride was a good chance to pick up some Spanish phrases. "Portense bien," the teacher said. Be good. "Sientense." Sit down. "Tengan Cuidado." Be careful.

It was like a party on the back of the bus. The passengers had put their newspapers down. Everyone was smiling, at the children, at each other. The young man in tight black pants who had been romancing the two pretty young women in front of him ever since the bus hit Columbia Road got carried away. "Will you marry me?" he said to one of the women, or maybe both.

The No. 42 Metrobus had been transformed.