"To tell you the truth, I'm a very ordinary person," says Quinn Cummings, who is trying to pass herself off as the average 10-year-old girl.

That is easier said than done when your name is on the license plate of the family Jaguar, when you managed to scare the daylights out of Johnny Carson on his own television show, and when your mother interrupted you on the set of "Baretta" Tuesday morning to tell you you'd just been nominated for an Academy Award as best supporting actress of the year.

Quinn who stole many a scene as saucy young Lucy in "The Goodbye Girl," with Marsha Mason and Richard Dreyiuss, and who had her ears pierced at the age of 8, says, "I'm very excited" about the Oscar nomination with the telltale weariness of a veteran, which, after three years as a working actress, she is. "She took the news very calmly," says her mother, Jan. "I, of course, was beside myself with glee."

To celebrate, Quinn and mom will go out tomorrow night for Quinn's favorite, a Chinese dinner. "You've never seen me go at mooshi pork," Quinn says.

Naturally one expects child actors to be insufferably bratty prima donnas. The last 10-year-old to get an Oscar was Tatum O'Neal, who soon took to high heels, deep cleavages and enough jewelry to choke a Gabor. At this point, though, Quinn seems enviably sober-minded and sensible. She is a wise child with knowlegeable brown eyes and a thoughtfully pudgy face. She is also extremely patient with adults.

"I do get hyperactive when I'm excited," she notes, and when she appeared on the Carson show a few weeks ago, she jumped about in the chair, snapped out brisk and witty answers and seemed absolutely to petrify the usually unflappable Carson. "I don't think he was petrified," Quinn recalls, sitting on the porch of her Hollywood Hills house. "I think he just needed somebody he could mug with. He's nice, really. I met him at the premiere.

"Speaking of which - at the premiere, here in California, I'd seen the movie so many times, that my mother arranged for me to go next door and see 'Star Wars' while the movie was on. I loved 'Star Wars.' I did find 'Close Encounters' more believable, though."

When Quinn is working, as she is now on the season's final "Baretta" ("I play a child psychic"), she is tutored on the set. Otherwise she goes to a normal sixth-grade classroom like a normal sixth-grader and beats up little boys like any other little girl.

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"I did beat up a couple of kids. They deserved it. I give 'em one punch and ran like the back of my head was on fire. Slow teasing is what does it to me. Like, I'm not too fond of my middle name - Louise - and one of the boys in the class kept going, 'Quinn Louise Cummings,' all day long, and he sits about two seats away from me and he mouths it at me, and that got me very angry.

"Then I found out his middle name, RAYMOND." She laughs. "I called him Raymond and he called me Louise and I called him Raymond again and he shut up."

Quinn's acting career began like that of many other Hollywood children - doing commercials. Unlike many other Hollywood children, the idea of an acting career was suggested to her by James Wong Howe, family friend and one of the greatest cinematographers in movie history. Howe has since died, bat his wife still walks her two dogs past the Cummings house. They are neighbors.

"My first commercial was for Future Floor Wax," Quinn recalls. "I just scuffed across the floor. Fifteen times! That's not as bad as when I did the Pop Tarts commercial. I did that one 32 times, and I had to take big bites, too.

"Commercials are fun to do, but some of them are awful dumb. I mean, how many 9-year-old people do you know who go, 'Oh lookie, it's a Polly Pretend Doll! A Polly Pretend Doll".

"I felt like such a fool."

Quinn isn't really the dolly type. "Once, for about two months, I was really hooked on Barbies. And then, I didn't play with them any more. I haven't even looked at them for about three months."

In 1975, she played the granddaughter of Sheldon Leonard on the CBS comedy series "Big Eddie." But that didn't last and she went back to the unemployment line. Oh yes, when Quinn's not working, she collects unemployment. She can recite her Social Security number without a moment's pause. At the unemployment office, her mother says, Quinn usually runs into five or six other child actors and they talk shop the way other kis might discuss the Super Bowl.

Quinn is not a burden on the state for long stretched at a time however - the longest was six months, when she thought of quitting the business, but reconsidered.

Other parts soon turned up, including one in the "$6 Million Man" Christmas show - "one of the world's worst scripts," when Quinn grumps. Now, if all goes well and the series is renewed, Quinn will join the cast of the girl adopted by the Lawrences.

Eagerness for work has its limits, thought, so that when Quinn was offered an audition for the movie "Pretty Baby," which deals with child prostitutes, she and her mother talked it over and decided to decline. "I just didn't want to go out for it," Quinn says matter-of-factly.

There are other career problems. Little girls do tend to get typecast as little girls. "I have brown hair, brown eyes and freckles," says Quinn. "If I had blond hair, blue eyes and freckles, I'd be even more into typecasting. Let's face it; they do stereotype you. Especially in those Disney movies. Have you ever seen a Disney movie with a dark-haired girl as the heroine No. Because they think blond looks 'wholesome.' I got down to the last two girls for a Disney movie once, and the other one was a blond, and she got the part."

Exasperated look. A slightly mopey shaggy dog named Ginger barks suddenly, but the little girl in the OshKosh B'Gosh overalls is not startled. "That's Ginger. She's my half-oyote, half-German sheperd bayyy-beee. Come here, Ginger." Quinn also has two hamsters and "Pooh, the fat cat of the Century." And she has two-much-older half-brothers both in their 20s; Quinn's father died last year.

She recalls making "The Goodbye Girl" as a thoroughly happy experienced except for the spaghetti-eating scene. "It took three days to shoot and I gained three pounds. It tasted AWFUL - no offense to the sound crew who cooked it. But it was AWFUL."

It took her "a little metnthol" to bring her to tears for the touching scene with Dreyfuss in the hansom cab; Quinn can't yet cry on cue. She is told the old story about how tears would be elicited from Margaret O'Brien on movie sets: O'Brien's aunt would tell her that her dog had just died.

"Personally, I don't think that something like that could have fooled me," says Quinn. "I don't think it fooled her, either. She was professional enough to go out there and shed tears. I heard that one time Margaret O'Brien asked the director, 'Should I let the tears go ail the way down my face or should I stop them half way?'"

In school, "I'm a real turkey at math," Quinn says, but reading in her best subject. If she watches TV, she watches "Rhoda" or "60 Minutes" - I love '60 Minutes'" - but TV-watching is restricted by her mother. So Quinn does the unorthodox thing; she reads.

"The best book I ever read was 'Little Women,' even though I did find Louisa May Alcott's other books pretty soppy. I have to admit it - she wrote something called 'Jack and Jill,' which I read and it's just heartrending that she could go from such a good book to such a loser. 'Jack and Jill -' (she assumes a prissy, melodramatic voice)' - Oh Jill! I have to leave you now for 25 minutes, will you be all right?' 'Yes, but it'll be hard to live without you.'"

Quinn flutter her eyelashes like Lillian Gish in distress.

"You know, Quinn has been very fortunate," says her mother. "She's met some of the bigger children in the industry and they've all been nice. She's had nothing but positive experiences in the business. I've run into a couple of mothers who were rather hateful, and a couple of directors who were really bad people, but that's part of life. She could meet that kind of person anywhere. She had a teacher once in public school who was just awful."

"She hated me," says Quinn. "She loved to blow a police whistle in my ear every five minutes."

But isn't it hard for Quinn to adjust to being a star?

"She isn't a star," Jan says. "She's a working child."

"Muhhh-thurr, that sound weird," says Quinn.

"Well, it's absolutely true."

"I know it's true but it sounds weird."

"Remember our dinner companion at the premiere party in New York - Claudette Colbert?" Quinn's mother asks her. "Now THAT is a star."

Quinn shrugs. "Have you taken the grand tour of the house?" she asks her visitor, and with that escorts him off through the house's several levels, which is what happens when you build a house on a hill. In the kitchen, Quinn asks. "You want an avocado? California unfortunately gets the heart of the avocado season. It is ALWAYS the heart of the avocado season. And I absolutely despise - OH!" Quinn has suddenly noticed that outside on the terrace there lies a fallen butterfly, motionless on a flat stone.

"Oh, what a beauty! Wait a minute." She rushes out, kneels down and looks at the quiet little thing. It has dark blue wings with bright yellow edges. Hollywood dogs are barking in the distance and dusk is making the view of Los Angeles below a musky grainy print.

"What a beauty," Quinn whispers again. She is watching the butterfly closely. "Alive? Ummmmm . . . No, I don't think it is. You know, we have a butterfly tree up there, and along about April, the thing practically ERUPTS which butterflies. Bright oranges and blacks.

"MAMA!" she calls out. "Look at this beautiful butterfly I found!" She has picked it up and is cradling it in her hands now, walking slowly toward the back door. "It died, poor thing, but it's still beautiful," she says, and it seems reassuringly evident at this moment that if Quinn wins the Oscar on April 3, it probably won't mean as much to her as the silent dead bug she is carrying into the kitchen.