Even in the dappled glade of Wolf Trap's Theatre-in-the-Woods, you can feel the heat and humidity of August at midday in the Washington area.

The shade of the trees gives some relief from the heavy air and insistent sun. But on the stage there's no cover and it's plainly hot. A group of schoolchildren dressed in heavy Swiss and German costumes smiles anyway and performs a folk dance for a photographer. The Christina Heimlich Dancers of Fairfax seem happy to do it again and again just to get it right for the picture.

At the Hunter Mill Country Day School nearby, another group of children cuts cardboard into small squares. It's a tedious job, snipping away until there are many large piles of them. After they're done with this, they'll move on to cutting thousands of short strands of yarn -- one after another after another.

And in Annandale, Judi Raaum answers the phone at the Annandale Chiropractic Associates cheerily, shuttles patients to the proper doctors and keeps the office in order. At the same time, she's also fielding calls constantly, organizing meetings and recruiting people to help out for a massive event that will happen in less than a week. With only a slight hint of tension in her voice, Raaum said, "It's a labor of love."

The cause of all this extraordinary effort is the International Children's Festival, a multicultural performance at Wolf Trap Farm Park from 11 a.m. to 4 p.m. Sept. 5 through 7, Labor Day weekend. Now in its 17th year, the festival is expected to draw more than 30,000 children and adults who will roam the meadows and woods of the performing arts park sampling the continuous entertainment on four stages. The festival this year features 36 performing groups -- from Canada, Taiwan and Bulgaria as well as from across the United States -- singing, dancing, and performing theater, magic and acrobatics. Clowns, puppeteers, mimes, and arts and crafts demonstrations and workshops, designed to delight the child and the childlike, also will be there.

Tickets are $6 for teen-agers and adults, and $4 for senior citizens and children ages 4 to 12. Admission for children younger than 4 is free.

The annual event is sponsored by the Fairfax County Council of the Arts, fund-raisers who provide the $100,000 annual budget and a cadre of high-level organizers. But the real drive behind the event seems to spring from the 1,200 volunteers, who lend time, talent and determination to pull the whole thing off.

The accolades for these backstage miracle workers come easily. "The festival wouldn't be possible without them," said Toni McMahon, executive director of the arts council. "I couldn't even imagine the cost if we had to hire all this help."

Her sentiment is echoed by festival chairwoman Fran Wright. "They work all year long on this with no reward except the joy of the children."

"Just incredible," said arts council President James C. McKeever Jr.

The Fairfax County-based Christina Heimlich Dancers have opened the festival for 16 years; Christina Heimlich was one of the originators of the festival. It is the only group that does not audition. Watching the children pose in their colorful ethnic costumes, doing a step or two of the dances, Wright said, "They really portray the spirit of the festival."

That spirit, though, is only one of the criteria needed to join the festival. Many groups send tapes to the festival committee, but only 36 were selected this year. The Bulgarian Zdravets will dance folk numbers, and Taiwan's Children's Folk Sports Presentation will tumble and jump, and the Canadian Wakami Wailers will sing national tunes.

Groups from the United States will include the Georgia Sea Island Singers, Indiana's Melchior Marionettes and the Red River Dance and Performing Company from North Dakota. Many acts are from Virginia, such as the Just Clowning Around clowns, the Fuzz and Stuffing Puppets and instrumental music from Nguyen Dinh and Family.

All groups pay their own way to the festival, though the international groups get a small honorarium to help. "Of course, we help them with lodging, transportation and anything else," Wright said. And in a new development this year, the festival has arranged with the Fairfax Board of Education to give the Bulgarian troupe a tour through the county school system and venues for performances for a few days after the festival ends. "We're very excited about this," McKeever said. "It continues the festival's spirit of international friendship and harmony."

The children at the Hunter Mill Country Day School also are integral to the festival, preparing basic materials for later use. The cardboard and yarn they cut will be used in the many arts workshops that will spread out on Wolf Trap's grounds. Professional artists will train 400 volunteers to run the booths, where children can learn everything from simple arts such as weaving and face painting to the more complicated Oriental brush-painting and Mexican flower-making. "The effort of the arts workers is apparent," workshop committee head Nancy Payne said. "It really rounds out the festival.

"The volunteers have to learn a lot in a little while, and it's quite impressive what they do."

Perhaps the center of all this is Raaum, who as volunteer coordinator for the festival plays a critical role. Though she's not in charge of all 1,200 volunteers, she recruits close to 1,000 of them from throughout the area.

"We have a core of people from the Boy Scouts and Girl Scouts and service organizations at local high schools," she said. She also went to 10 area corporations to attract help and received an overwhelming response from workers.

Besides working in the workshops, Raaum's volunteers hand out programs, prepare informational packets for performers and other volunteers and evaluate all the 100 performances for future selections. "We do just about anything," Raaum said.

Her people will have an orientation on Aug. 31 at Wolf Trap, a kind of dress rehearsal for volunteers. "We want it to go as professionally as possible," Raaum said with clear anticipation in her voice. She has put a few hundred hours all year working on the event and will take next week off to devote to the festival.

What gives her that kind of determination? "Well, I guess the fun I have doing it." But there are other rewards for herself and other volunteers.

Raaum remembers a woman who brought her son in a wheelchair to the festival, and wanted to leave him with Raaum while she did her volunteer job. Raaum gave the teen-ager, who had cerebral palsy, a job handing out programs at the gate. "You should have seen the smile on his face by the end of the day," she said. "He really felt great for having done something."