The timing could hardly have been better for the launching of the Beethoven Pops Orchestra, Washington's answer to Arthur Fiedler. Washingtonians starved for the sound of a live symphony flocked to the Presidential Ballroom of the Capitol Hilton yesterday afternoon, bringing their children with them, and everyone from toddlers to the Geritol generation seemed to enjoy the program thoroughly.

Children wandered about the large room, some carrying white balloons, or sat in front of the orchestra's platform, clustering around Captain 20, a television personality who sat on the floor with them.

Some of the children played along on imaginary keyboards as Glenn Sales, a phenomenal teenage pianist, played through Rachmanivov's Second Concerto, and they bounced with excitement when the typewriter soloist began playing in Leroy Anderson's "The Typewriter" and when the brass secion of the orchestra got up and marched around the room during Sousa's "Stars and Stripes."

On the podium, under a crystal chandelier, Richard Weilenmann conducted a medium-size orchestra with many faces familiar to regular NSO patrons. The room, which has hosted many dance bands in living memory but few symphony orchestras, turned out to have bright, lively acoustics that made the ensemble sound larger than it was and gave the bass and percussion an impressive presence. The playing was spirited and very stylish (particularly in the Strauss "Emperor" Waltzes), though the slow movement of the Rachmaninov might have benefited from more rehearsal.

A second program, devoted to Viennese music, is tentatively scheduled for early January.