IN SOUTH Carolina recently, Ella Fitzgerald performed for 13,000 children. Afterward, one little boy told her he was disappointed that she didn't break the glass. She realized: "To these kids I wasn't Ella Fitzgerald, but the lady who breaks the glass on television."

Those ads were for Memorex Tape ("Is it live or is it Ella?") and they celebrated a voice that is the most supple of instruments, from the soaring high notes that cap her virtuoso scat pieces to the resonant low tones she has discovered only in the past few years.

She has since sung a popular commercial for Kentucky Fried Chicken. Doing commercials is a change of pace for the winner of countless music awards and polls, the bearer, at 65, of more than a dozen honorary degrees from such schools as Dartmouth, Howard, Boston University and Washington University in St. Louis, and one of the five jazz musicians honored with Kennedy Center Awards for contributions to American culture. Tonight she opens the Kool Jazz Festival at the Kennedy Center.

Ella Fitzgerald was in Baltimore last week to accept an honorary degree from the Peabody Conservatory, having just sung in New York at the Friars' Club dinner for Elizabeth Taylor, sharing the dais with Frank Sinatra, Dinah Shore, Joe Williams and other stars.

"We were doing 'Night and Day,' and I was singing these little ad lib things. I could hear Frank and Joe behind me going, 'Yeeaah!' Well, I thought I was really singing."

Sitting straight-backed on a sofa, Fitzgerald, in a blue suit with a blue, green and purple blouse--was trimmer than she has been at any time since the 1930s. An eye condition that began troubling her in 1971 has stabilized. Her fame is secure. She talked about her life.

She never knew her father. As an infant, she moved from Newport News, Va., with her mother and Portuguese stepfather to Yonkers, N.Y.

Her mother liked to sing, classical music mostly, but she also had records by the blues singer Mamie Smith, the Mills Brothers (with whom Fitzgerald later recorded) and Connee Boswell, the singer who was her inspiration.

But as a child, Fitzgerald was most impressed by a rubber-limbed dancer named Snakehips Tucker.

"People in Yonkers thought I was a good little dancer, so that became my ambition. They used to tease me and call me Snakehips Ella. I entered an amateur contest at the Apollo (the theater in Harlem) as a dancer, but when I got out on stage and saw all the people and the lights, I guess I lost my nerve. The guy said, 'You're up here, do something!' The first thing that came to mind was Miss Connee Boswell. I knew her records of 'The Object of My Affection' and 'Judy,' so I sang those songs and won the contest by imitating her.

"As a kid, I didn't pay much attention to music. My mother hired a man to teach me piano; the lessons cost $5 and we were poor. The teacher had slit the skin between his fingers so that he'd have a wider reach. I was so fascinated listening to him talk and play that I hardly learned a thing."

She grew up in a neighborhood of Italians, Portuguese and blacks. For extra money as a girl she was a courier in a numbers game, and worked as a lookout for what she has referred to as "a sporting house."

The first time Fitzgerald won a singing contest, alto saxophonist Benny Carter was in the audience. He had her sing for bandleader Fletcher Henderson, who was not terribly impressed. At age 16, she joined Chick Webb's band--after the drummer agreed to become her guardian on the road. Fitzgerald credits Webb with helping her to forget Connee Boswell and forge her own singing style.

"I began trying to sing ballads, and he took the tempos down gradually without my even noticing it. I had never really studied music, so whatever came out of me, that's the way it was. What I know, I've learned from the bands I've worked with--Duke Ellington, Count Basie, Dizzy Gillespie. If the musicians like what I do, then I feel I'm really singing. They say I have a good ear, which was enough to start me. Then I had to experience different things, to learn how to tell songs like stories."

While with the Webb band, Fitzgerald married a man on a dare; he bet that she wouldn't, and she took him up on it. The marriage was later annulled. (In 1948, Fitzgerald married bassist Ray Brown, and they adopted a son, Ray Jr., who now plays drums and guitar with a band in Seattle. Brown and Fitgerald were divorced in 1952 and she has not remarried.)

Her 1938 hit record "A Tisket, A Tasket," cut with the Webb band just after her 20th birthday, brought her national attention. When Webb died suddenly in 1939, Fitzgerald became the band's nominal head, though in fact it was directed by others. ("They let me conduct one number each show to make me feel that I was the leader.") Then war broke out and more band members were drafted than could be adequately replaced. The Webb band was dissolved in 1941. Fitzgerald then toured with the Ink Spots vocal group before going out as a single.

"I learned scat singing from Dizzy Gillespie while I was on a tour with his big band. Listening to Dizzy made me want to try something with my voice that would be like a horn. He'd shout 'Go ahead and blow' and I'd improvise. We did 'Lady Be Good' on the 'Make Believe Ballroom' radio show. The people at Decca heard it and had me record it. Dave Garroway, God bless him, played that record so often on his program in Chicago that I got to work every theater in the city. Bopping was a different thing and everybody wanted to hear it."

Norman Granz, impresario of the touring Jazz at the Philharmonic packages, admired Ella's bopping and invited her to join his all-star caravan in 1950. As things turned out, he changed the course of her life.

"He got the idea of the songbook albums--Porter, Rodgers and Hart, Ellington, Berlin, Gershwin, Arlen and Kern. It was like a new beginning. Now, in addition to the jazz singing, I had something to offer people who wanted to hear the pretty songs. I was learning something new and becoming someone else. I don't think we ever stop learning in music."

These days, Fitzgerald travels less than half the year, but grows restless after a few weeks at home. Her biggest interest away from music is charity work. "I'm a glutton for anything that involves kids. I donated three nights of performances to benefit a nursery school that's named after me in Watts. Oscar Peterson and Basie did the same, and Henry Fonda, God bless him, made some beautiful pictures that were sold for the school. We raised enough to construct the building, and each year they add onto it."

Although some reviewers disagree, Fitzgerald says aging has not taken too great a toll on her vocal equipment.

"Of course," she admits, "as you get older, you start worrying about your vibrato and all that. But by taking some of my songs down a tone or two, I can do a full show without straining. And in the past few years, I've developed some low notes that I've never had before."

Otherwise, turning 65 hasn't affected her in the slightest.

"I enjoy what I'm doing now more than I ever have. Let's face it, after all these years, most other people have stopped singing. Some of them are popular this month, next month you don't hear anything about them. I feel I have a lot to be grateful for."