While his sister got her face painted, Brendan Schoen tried a scientific experiment yesterday. Squeezing an eyedropper for all it was worth, Brendan, 3, watched as a little dome of water rose above the rim of the cup he was holding and created a meniscus.

While Brendan and a line of boys toyed with microscopes and magnifying glasses, his 6-year-old sister Jenny was, like many of the girls at the 15th International Children's Festival, trying to look like Madonna.

Wolf Trap Farm Park turned into a reservation of children this weekend as thousands of youngsters and their parents crowded lawns and picnic areas, turning meadows into vast parking lots for buggies and strollers. From Saturday through today, children have been in command. And parents from around the region went where their impatient youngsters took them.

"I want to see the funny men," said Jennifer Doherty, 2, of Herndon, pleading with her parents to take her to where clowns were painting bright flowers and shiny cars on children's faces. "I want purple face."

The festival brought performers from 21 states and 23 foreign countries to play under the big canvas tents that had been set up amid the greenery of the park. Hundreds of volunteers had spent the best part of a year putting the festival together.

Children and adults turned their feet to everything from square dances to the ritual steps of a Cambodian hill tribe. A group of Japanese women living in the United States, known as the Minyo Dancers, held a capacity crowd spellbound in one of the meadows. Draped in crisp white yukata, or summer kimonos, tied with distinctive wide green belts called obi, the dancers looked like porcelain dolls moving in mechanical rhythm.

There were arts workshops where children could make puppets, learn that a meniscus is a curved bulge of liquid that does not spill when it seems that it should, or create abstract art with a computer.

For the boys, "Science and Illusion" appeared to be the biggest draw, while basic face painting had the girls lined up among the thick and graceful oak trees.

"Everything was good," said Jessica Rose, who said she was "8 1/2, almost 9." But she said the best part of the day was getting a Girl Scout to help her manufacture a multicolored rose from a bunch of Kleenex. Jessica wore the flower in her hair as she sat and watched the troupes from around the world perform in the Filene Center.

Although the workshops drew long lines and eager participants, the performances were the highlight for many of the people.

"How do they keep from throwing up?" asked Brad Pribbe, 7, of Springfield, as he watched the Tennessee Children's Dance Ensemble whirl and stomp to a powerful combo of harmonicas. "I can't even look at it without getting dizzy."

For some of the young performers it was a question that had a great deal of relevance. When asked if he was nervous as he filed on stage to perform in front of several hundred people, Josh Lauer, a member of the Singing Boys of Pennsylvania, answered with one emphatic word: "Absolutely."

Other youngsters were more nonchalant about their big break in front of an audience, especially those who were seeing the United States for the first time.

"I am so very happy," Chung-Cheng said several times in halting English before he went on stage to show Americans what the people of Taiwan already knew: that he was one of the finest rope jumpers in Asia. Chung, a 15-year-old in the ninth grade, has toured several countries showing his prowess with the rope, but this was his first time at the Children's Festival.

Although the mood was upbeat at the festival yesterday, the frayed nerves of some parents began to show as the afternoon wore on.

"If this kid gets any more paint on me I'm going to kill him," said Cheryl Brinkel of the District as she clutched her son Jon in one hand and a box of chicken in the other. "Thank God the summer is over."