Marjabelle Stewart terms herself that all-important ''other voice'' when it comes to teaching manners to children and teen-agers. That is the voice, she explains, of an impersonal but interested outsider, which children hear and heed after they have successfully tuned out instruction from their parents.
At one point she may have been a voice crying in the wilderness (manners took a beating during the '60s, she admits), but no more. Manners are in again in a big way, she says, and people are acknowledging their importance.
''Everyone has long since grown tired of slobs and rude behavior and rebelliousness,'' she declares stoutly. ''We had a revolt against manners, but a new revolution is now reinstating them. People want to know what the social standards are again and yearn for the form, tradition, and orderliness that come from observing them.'' Occasionally she has to reassure someone that having manners doesn't make one an oddball, nor do manners indicate that one is trying to be highfalutin or is putting on airs.
Mrs. Stewart began her etiquette-teaching career in Washington, D.C., over 20 years ago when she opened a little school called ''Mrs. Young's Classes for Young Ladies,'' which attracted youngsters from government and diplomatic families. She has been expanding her teaching program through classes, books, demonstrations, and lectures ever since.
Now she lives in Kewanee, Ill., with her lawyer husband and children, continuing to devise new ways to drop her manners message on receptive ears. Once a month she goes into Chicago to teach her method to franchised teachers, who then conduct classes in department stores in 800 cities around the country or sponsor their own sessions.
The three books she has written with Ann Buchwald, the wife of Art Buchwald, the columnist, continue to be classics for instructing the young. These include ''White Gloves and Party Manners'' for girls (Washington, D.C.: Robert B. Luce Inc.); ''Stand Up, Shake Hands, Say 'How Do You Do'' ' for boys; and ''What to Do When and Why'' for young teens (both published by David McKay Company, New York). Her own ''The Book of Modern Table Manners'' has been published by St. Martin's Press in New York. This fall Signet will produce another of her books on teen manners in a $2.98 paperback edition.
Mrs. Stewart has also launched teaching sessions, in cooperation with the Ritz-Carlton and Whitehall Hotels in Chicago, the Copley Plaza in Boston, and the Waldorf-Astoria in New York. These classes teach young people how to deal with the complexities of hotels, including front desks, concierges, room service , hotel dining rooms, and the intricacies of formal dining out.
''We take the mystery out of hotel visits and help young people feel at ease and comfortable with all the situations they might encounter,'' Mrs. Stewart says. ''Children are dining out with adults more often these days, and a child enjoys this experience fully only if he knows what is expected of him and what the rules are. There is no excuse for children being embarrassments to their parents because they don't know any better.''
Her licensed classes, she says, have reached thousands of youngsters from families of all economic and social levels. Plenty of parents and grandparents, she says with a laugh, have expressed gratitude for her help in teaching their youngsters.
''I always go on the theory that I am reminding them of things their parents have already taught them, but just making it all seem like fun and something they really want to do,'' she says. ''I strive to give children an overall positive attitude towards good behavior by telling them that manners are simply polite, kind, and considerate ways to behave with others.''
Certainly Marjabelle Stewart is a master at coaxing good behavior out of even the most reluctant young client. Her voice is soothing and persuasive. Her manner is smooth and graciously respectful of each child. She tells her charges that she knows well what shining, wonderful persons they truly are, and that good manners simply add the polish to that shine.
One afternoon recently Mrs. Stewart steered a group of students from the United Nations International School through the fine points of dining etiquette. She told them how to enter a room, then how to greet and be greeted by the hostess and other guests. She explained the proper placement of silverware, china, and crystal in a mock place setting and how they were to work their way from the outside in, as the courses arrived.
The setting was the dining room of the Royal Suite of the Waldorf-Astoria in New York. The table was magnificently set with exquisite silver, china, crystal, candles, and white linen. Waiters hovered. Courses arrived in impressive procession. Mrs. Stewart, presiding elegantly in a long gown, purred softly, ''Now don't snap your napkins loudly, but quietly half open them and lay them across your lap. And at least try all the food. Even if you don't like it, move it around on your plate. Don't just say 'yech' and put your fork down.''
This auburn-haired woman, whom Time magazine dubbed ''a crusader for couth,'' showed the students how to sip water gently, how to pat their mouths with the napkin, how to engage in polite table conversation. She warned them about losing their napkin on the floor and having to go diving under the table to find it. And she told them not to flap their elbows as they maneuver their knives and forks: ''Let's not look like we are flying. It takes real art to stay within your own dining space.''
She showed children, aged 7 to 14, how to break rolls by thrusting their thumbs into the center of the roll (crumbs over the butter plate, please, not on the tablecloth) and then pull off small pieces for buttering. She told them to eat soup by ''dipping away, leaning forward, and sipping.'' At this session, a small Japanese girl even instructed the others in how to dine with chopsticks.
Mrs. Stewart admits she has to remind her own children, who aren't always models of etiquette.And it bothers her 14-year-old son, Billy, to hear his mother referred to as ''the manners lady.'' ''But we are all learning something new every day about flexibility, and how to cope with new situations with some grace, so we are making progress,'' his mother says.