"Now when I sing 'Ay, yi yi yi,' " said tenor Manuel Melendez, "you sing it after me."

Thirty children stared up at him. They sat in a Mexican village scene at the Capital Children's Museum, some perching on the tile fountain, some lining the little market square before the cardboard church.

Melendez, of the Washington Opera Company, was here to tell the world about a concert to benefit Operation Mexico Nov. 18 at the International Club.

You remember Operation Mexico. That's the three British cyclists who are trying to raise $50,000 to help victims of the 1985 Mexico City earthquake. Though Mexican officials say more than 95 percent of the damage has been repaired, there is this housing project that is still 22 houses short of its goal, and it is for this that the cyclists, with single-minded ferocity, have been touring the country.

Now, after four months of concentrated finagling among promotion-weary Washingtonians, they have lined up Melendez, Suzanna Guzman and other opera singers, plus part of Mexico's Ballet Folklorico and two local mariachi bands for a gala evening of opera, Latin music and Broadway songs.

Food is being donated by the International Club and La Fonda restaurant, and the sponsors -- ranging from the Kennedy Center and the Washington Opera to the Pan American Development Foundation and the Hispanic Institute for the Performing Arts -- are looking for 250 guests at $50 each.

Operation Mexico also has been running several nightclub events featuring Big Bang Theory, New Keys and B-Time, playing at d.c. space and the East Side club. And, just for variety, there was a Celtic-flavored benefit blowout at Kelly's Irish Times.

Later in the fall the group will hold a TV auction starring the actual autographed drumsticks of U2's Larry Mullen, the actual handkerchief with which opera star Placido Domingo as Otello strangles Desdemona, and other icons. Domingo has been the group's guardian angel ever since he netted them $40,000 in a London concert. Last time he was at the Met in New York, the group drove up to see him and back the same night.

But at the Children's Museum, the cyclists, Adam Baines and Alan O'Donnell, were concentrating on their media event. Someone was handing out sombreros and tiny serapes ("there should be 20 kids -- that's what we ordered," deadpanned O'Donnell). Someone was coaching 9-year-old Melissa Best to say, "The children of Washington say to the children of Mexico City, 'Hello! Hola!' " Someone was translating the song "Cielito Lindo."

"It goes, 'canta y no llores' -- that means 'sing and don't cry,' " Baines said. He and O'Donnell have been swotting up on their Spanish, preparing for their bike ride into Mexico. They have cycled all the way from Nova Scotia in the cause, for which they quit their jobs. The third rider, Deborah Green, is in Mexico now, visiting the housing project.

The Spanish lessons came free, courtesy of Berlitz, and recently when Baines broke his glasses, Embassy Opticians gave him new ones. Life has been like that here for the cyclists. For weeks and weeks they have been given free lodging in the home of architect Roy Mason.

"Ready!" shouted the TV crew.

The pianist played. Melendez sang.

"Ay, yi yi yi," he sang, and the kids started to come in but he held them off with a lifted finger.

Then he got to the chorus. "Ay, yi yi yi," he sang, "canta y no llores."

"AY, YI YI YI," the kids sang, "CANTA Y NO LLORES ...