A small virtuoso children's chorus from Puerto Rico inaugurated the Kennedy Center's newly refurbished Concert Hall yesterday morning.

The Coro de Ninos de San Juan performed with considerable discipline and finesse: in fact, the group of about 20 young people and their occasional accompanists on guitar and piano took on a program which many an adult professional group might hesitate to try.

The chorus used to sing regularly for Pablo Casals at his home, and performed 16 works from memory at the Concert Hall with ease. Hardly anything seemed to faze them.

The opening works of the concert, which began the Center's first International Youth Choral Festival, were mostly by Renaissance composers - Victoria, Passereaux, Victoria and Lasso. The children only had difficulty with the sole romantic selection, Brahms' "Marznacht," in which the chromatic passages seemed flat. But the expert conductor, Evy Lucio de Llobet, quickly set things straight.

Later there was lighter music, with Aaron Copland's wry "I Bought Me a Cat," bringing down the house.

Also the program was the Page High School Choral Ensemble of Greensboro, N.C. They did well, but were both less ambitious and less disciplined than the Puerto Rican group.

Three other choruses will perform free during the festival. All will sing separately and then at 7:30 tonight will in some spirituals under conductor Jester Hairston at the Concert Hall.

The choruses were selected on the basis of quality and, as the Center's education director, Jack Kukuk, puts it, "their ability to pay their way." It's costing the Center $20,000, which comes from its Corporate Fun for the Performing Arts.