Thousands of youngsters are expected to blanket Wolf Trap Farm Park this weekend for the quintessential children's performing arts festival, complete with storytellers, dancers, puppeteers, magicians and make-believe. Grown-ups are invited, too.

Organizers of the 15th annual International Children's Festival expect as many as 20,000 people to attend the cultural arts celebration, which will feature a hefty helping of young talent from 21 states and 23 foreign countries. Performances will include modern American dances; Greek, Indian and African dance; gymnasts; mimes, and singers.

More than 1,300 performers -- many of them between the ages of 6 and 19 -- will participate in the three-day festival.

The Contemporary Jazz Dance Theater of Reston and the Mekong Performing Arts dance company of Falls Church were two of several Northern Virginia groups chosen from nearly 300 groups vying for a slot in the festival.

Local auditions for this year's festival were held last spring at Fairfax High School. Twelve panelists judged performances on family appeal, quality, stage presence, educational value and potential for audience participation. Out-of-town groups sent videotapes of their proposed routines.

Fran Wright, festival program director, said the Reston and Falls Church dance companies showed outstanding "discipline and dedication to their performance" at the auditions.

"Although they're amateurs, they come across as professionals," Wright said. "The Conservatory Jazz Dance Theater are really skilled dancers. We could just sit and watch them all day."

Founded by Herndon resident Diane Yates-Biggs, the jazz dance theater began in 1981 as the Floris Fancy Dancers and quickly grew into the Herndon Youth Dance Company, sponsored by the Herndon Department of Recreation and Parks.

But Yates-Biggs said her young dancers needed an environment more conducive to practice and learning their art than the town's recreation department could offer.

"We needed mirrors, bars, dance room and a dance floor," said Yates-Biggs, who also is a speech and hearing clinician for Fairfax County schools during the academic year.

In 1984, Yates-Biggs found more spacious and equipped quarters in Reston's Sentec Building, where the Conservatory Ballet Ltd. is housed, and established her jazz dance theater for youngsters between the ages of 8 and 16. The classes meet Mondays and Thursdays for two-hour sessions. The fee is $100 for four months.

The upcoming performance marks the third year the jazz conservatory has participated in the festival. Yates-Biggs plans to spotlight her budding ballerinas' grace and agility with a jazz routine called "Walk to Sunshine." The choreography "plays up the technical ability of the older girls . . . it has a lot of balletic influence," said Yates-Biggs, a native of Richmond.

The young dancers also will perform two other original routines: "Margarita," a snappy calypso-style number that features the group's younger members dressed in red, blue and yellow flaired skirts, and "The Boy," a jazzy, upbeat dance performed by three girls dressed in tomboy-like outfits.

Performances by the jazz dance group are scheduled for Saturday at 11:45 a.m. and Sunday at 12:15 p.m.

The Falls Church-based Mekong Performing Arts also will be returning to the festival for a third time. The 25-member dance group was begun in 1979 as an entertaining way to teach young Vietnamese children about their heritage. The group's repertoire includes traditional Vietnamese dances dating back to the 13th century.

Thu H. Bui, the group's founder and director, said he also was interested in mixing his Vietnamese culture with "the richness and diversity of the American culture."

At this year's festival, the Mekong Performing Arts will perform the "Peacock Dance," in which 16 young dancers imitate the graceful patterns and movements of a peacock. The group is scheduled to perform Sunday at 3:15 p.m.

Bui, who came to the United States from Saigon in 1973, is an assistant principal at J.E.B. Stuart High School. Meanwhile, he dreams of opening a Vietnamese performing arts school and teaching children the ancient dances and songs of his ancestors.

Bui said the dance group usually charges $200 for a performance; proceeds are used for costumes, transportation and refreshments during practices, which are held Sundays at St. Paul's Lutheran Church on Leesburg Pike.

The international festival is sponsored by the Fairfax County Council of the Arts, a private, nonprofit organization that produces cultural programs throughout the county. Council officials expect to raise $90,000 this year, two-thirds of which will cover the festival's operating costs.

The festival will be held Saturday, Sunday and Monday from 11 a.m. to 4 p.m. at Wolf Trap, which is located off Rte. 7, four miles west of I-495.

Admission is $5 for teen-agers and adults, $3.50 for children aged 4 to 12 and free for those under 4. Parking is free.