"Do you think this little girl can play the bass?" world-renowned Keter Betts asked 75 preschoolers seated on the floor of Wolf Trap's Barns. And then, his audience's skepticism notwithstanding, Betts placed the pigtailed tot's fingers on the strings of his upright, said some magic words and, with the spirited assistance of pianist Hilton Felton and drummer Bertel Knox, the trio -- oops, quartet -- was off and running on the Sesame Street theme.

Such occasions occur frequently under the auspices of the Wolf Trap Institute for Early Learning Through the Arts, a program of music and dance performances for preschool children from low-income families throughout the Washington area. Partially funded by Head Start, the programs are tailored to meet the developmental stages of preschoolers. Early childhood learning centers interested in attending such sessions can apply by writing the Wolf Trap Institute, 1624 Trap Rd., Vienna, Va. 22180.

After the little girl finished her stint on the bass one recent weekday morning, it was a delighted young man's turn to take his place at Knox's drum set. Knox called up a half-dozen volunteers, selecting for each a hand-held percussion device from his "bag of goodies."

"I see professors," announced the veteran drummer as he peered out into the mass of 3-, 4- and 5-year-olds. "I see scientists. I see principals of schools. We are jazz musicians."

With a front line of percussionists raring to go, Knox asked what school they were from, and so informed, announced, "Arthur Capper's Junkyard Band will now perform -- one, two, three, four!" And to Knox's finger-snapping pulse the kids carried off an amazingly coherent and rhythmically sound selection.

Then it was pianist Felton's turn to do his thing. He fashioned a "Name That Tune" session, offering for consideration blues- and boogie-flavored versions of the "Mister Rogers" and "Bugs Bunny" themes. It was no contest, he discovered, for his audience screamed out the correct identifications upon hearing the first notes. Then he invited the children to join in "We Are the World," and their undeveloped voices lent a poignancy to the song's message.

Finally, to the children's glee, Betts brought six or seven of the children's teachers and Clennie Murphy, Head Start director, to the front of the room. From a tattered bag, he produced a coffee can with beans in it, a saucepan and wooden spoon, an eggbeater and other makeshift instruments.

"Don't giggle until they turn their backs," Betts advised his giggling audience. And with Knox and Felton in their places, he led his "kitchen" chorus through a series of solos that had each in turn strutting along in front of the bandstand.

"Sometimes kids will come up to you and say, 'I've seen you before, you came to our school,' " said Betts, who spends months on the road with Ella Fitzgerald, and for the rest of the year does 50 or 60 programs for audiences ranging from 25 to 500 at area schools under the auspices of the Washington Performing Arts Society, the Montgomery County Public Schools and other organizations.

Betts has been involved in jazz education for young people for about 15 years. In 1979 he organized a Blues Alley evening that brought to attention the then-13-year-old drummer Terri Lyne Carrington. who now tours with trumpeter Clark Terry. Also in the program that evening was the now internationally known combo Pieces of a Dream, which recently released its third album.

"It's just been beautiful," says Knox, a longtime Betts associate who also has toured with Pearl Bailey and recorded with guitarist Charlie Byrd. "Sometimes you see these kids and they point me out and say, 'That's the fellow who played with the group at school.'

"And that makes you feel good because you know that they paid attention. And they're asking 'When are you coming back' and how can they hear that music again. And their teachers see me working in a nightclub and say, 'Didn't I see you in our school?' and go and rave about the program.

"That's my motivation -- these kids are my motivation. That's why I'm thoroughly, 100 percent involved in the program, really.