When Director Mimi Turner of the Mount Vernon Community Children's Theater found her cast backstage at one point in their rigorous, four-day-per-week rehearsal schedule, they were "literally rolling around on the floor, biting each other's ankles," she says.

Ten minutes later they were up on stage, mimicking the ennui-ridden young aristocrats in "The Importance of Being Earnest" and delivering Oscar Wilde's biting aphorisms with sophisticated timing.

Such are the extremes of children's theater, one of the livelier arts. Hals a dozen children's troupes in this area take kids as young as six through the acting business, giving them what director Lenore Riegal of the Trinity Players Children's Theater calls "real-life skills. It helps in meeting people, talking with people talking before a group."

"I guess it's helped with oral reports and stuff like that," muses 14-year-old Julia Cline of the Mount Vernon troupe, but she sees a much more practical side: "I just moved here two months ago, and I came to this play and made all these friends," she says brightly.

There may even be monetary value to this lively art. Trinity Players graduates are currently starring in the Ford's Theater production of "A Christmas Carol" and a new NBC series called "Our House"; other troupes have had members in local theater and TV casts.

But to be members, they first have to get past the auditions -- experiences that three- time-star Alice Hogan, a 12-year-old with the Children's Theater of Arlington, says can be "pretty disappointing."

"It's a hard way to learn that life is unfair," says manager Marcy Ubois. "We try to relax the kids and make it fun." In addition to having the children read lines from the play, the Arlington group usually calls for "some sort of improvisation, sometimes some mime," she says.

"They usually ask us to be animals," says 12-year-old Sean Roberts, who's been in six productions. "Once, they told us to be lumps of clay turning into something." Watching other kids cope with these orders "gives you ideas and sometimes you can do better," says Hogan. "But at least you've learned something," she believes.

The directors, in turn, watch the children, looking for certain heights and types, voices that project and "good behavior," says Ubois. "So if you don't make it," says young star Hogan, "it's probably because you don't fit the type, not because you can't act."

Once chosen, the group settles into six-to eight-week rehearsal schedules that require "a real commitment," says director Ubois. Fourteen-year-old Judd Crapa, who played Earnest in the Mount Vernon production, confesses that homework sometimes suffers: "You go to school, you go to rehearsal, you go home and eat dinner, you do your homework, you go to bed," is how he describes the rehearsal day, saying that when the play ends, he's planning on some "lazy days at home."

Children with lazy days available sometimes do better than the busy soccer, French and gymnastics stars of school, says Trinity's director Riegal. "You'd be surprised at how ordinary these kids are. Then they get up on stage, and something magic happens," she says.

The children involved are much more specific on what kinds of kids do best: "Someone who's spazzy, but not overly spazzy," explains Hogan -- using a phrase that apparently means someone who is enthusiastic, but not silly.

"Someone who's not a nerd," states Roberts, who adds that "it can get pretty rough MONTGOMERY COUNTY RECREATION DEPARTMENT, Wheaton; 468-4172.

Too late to sign up for children's theater classes now, but next summer the department's staff will teach theater to a thousand kids in classes, day and resident camps, and performance labs. VIRGINIA ASSEMBLY CHILDREN'S THEATER, Springfield; 978- 9536.

Registration for a five-month session begins in January. Classes, given for a fee, lead to a production. CHILDREN'S THEATER OF ARLINGTON, Arlington; 558- 2161.

Auditions for the "Ransom of Red Chief" coming up January 17 and 18 for nine-to 14-year-olds. FAIRFAX COUNTY CHILDREN'A THEATER, Fairfax; 691-2671.

A production of "In One Basket" runs December 3 through 6. Auditions for the spring production will be held January 4 and 6 for Fairfax County and City residents, ages ten to 18. No play has been selected at this time. McLEAN CHILDREN'S THEATER, McLean; 790-9248.

Its premiere production, "House at Pooh Corner," will run December 11 and 12. No spring show is planned this year. MOUNT VERNON COMMUNITY CHILDREN'S THEATER, Mount Vernon; 780-5813.

A Shakespeare workshop will be held in January, as well as auditions for "The Wizard of Oz," all ages welcome.y rough backstage" if you don't fit in. "I've come down on people who were too weird."

But for the Mount Vernon group, the enemy is somewhat larger. "Parents!" they answer in chorus, when asked for the worst part of children's theater. "They keep telling us we're almost adults, and expect us to behave like adults," sdays Crapa, who admits that the troupe often got "pretty obnoxious" during rehearsals. But "This is the Mount Vernon children's theater, get it? It's supposed to be for children."

High expectations are common among the adult supporters of these groups, typically parents who have been roped in to do makeup and costumes and sell tickets. The Arlington group gets its costume and set designers from Arlington County; the rest struggle along on ticket sales and community support. Some, like the Bethesda Academy for the Performing Arts, charge their troupers a fee "to cover costumes and scenery," says a spokesman.

All these groups tend to produce what Grapa calls "kiddie shows -- Winnie the Pooh, stuff like that" that appeal to audiences composed of the actors' friends and relatives. But while the quality of a few productions can best be described as "cute," directors strive toward something bordering on legitimate.

"We try to be professional, to give the children an experience in professional theater," says Arlington's Ubois, although her casts go out to the lobby after a show to sign autographs -- something legitimate theater would never allow. "But they love it," she justifies; "that's their reward."

"I like it when you get up on stage and make a fool of yourself, and the audience laughs with you," says 12-year-old Stephen Arner, Algernon in the Mount Vernon production. "That's what's fun."

In children's theater, like its parent, they do it for the applause. Here's what's coming up in area children's theater groups: WASHINGTON TRINITY PLAYERS CHILDREN'S THEATER, Georgetown; 965-4680.

"Cinderella" opens December 3 and plays through December 12. Auditions for "Charlie and the Chocolate Factory" will be held January 8 and 9 for six-and 16-year-olds. MARYLAND ADVENTURE THEATER, Glen Echo; 320-5331.

This group offers a wide variety of classes for ages five and up, some leading to performance. Fee. BETHESDA ACADEMY OF THE PERFORMING ARTS, Bethesda; 469-6703.

Classes in various theater arts; children auditioned from classes into four different troupes. Fee. JEWISH COMMUNITY CENTER OF GREATER WASHINGTON, Rockville; 881-0100, Extension 340.

Children's classes in creative drama for first through fourth-graders start in February. There's also a class in American Musical Theater for 12- to 15-year-olds, and a Young Actors class for 11- to 15- year-olds.