When Christian Badea picks up his baton and starts leading the National Symphony, he instantly becomes a dynamic, high-voltage conductor of obvious authority and know-how.

In his first appearance with the orchestra at yesterday morning's Young People's Concert in the Kennedy Center, the NSO's new EXXON/Arts Endowment conductor of necessity chose his program largely from music the National Symphony has played recently. But with skillful choices and the addition of such unusual items as the opening movementof a Handel Oboe Concerto, beautifully played by Sara Watkins, he put together a package that illustrated explicitly the theme of the day. "The Key Is . . . Rhythm."

The packed house of junior and senior high school students gave out big whistles and cheers for the Shostakovich Festive Overture, the same for Leonard Bernstein's music from "On the Waterfront," and clapped at the name of Marlon Brando, one of the stars of that film. Mozart won whistles of approval, Wagner, in the Ride of the Valkyries, pulled in a higher rating. Like their parents, young listeners follow a decibel chart in awarding their enthusiasm.

Badea did his own introductions, making special friends between the audience and the individual players in the percussion section as each one showed his specialty.

The handsome young conductor may be new at this game: in any case he must resist the temptation to misinform for whatever reason. For example, you can bet that Wagner never forgot a single word of the text of his Ring Cycle, though Badea suggested that he did; and it is quite easy to tap your foot to a vast amount of 20th-century music despite his comment otherwise.

Most of all, if he plans to contiunue doing his own talking at these concerts. Badea must improve his mike technique. Much of what he said was lost out in the house. He is too good a conductor not to make the fullest impact in every aspect of his work.

The program began 15 inexcusable minutes late.