When the 198 handicapped children at Key Center in Fairfax County left school last Wednesday, their playground was nothing but an asphalt patch with two old tires and a rusty tricyle.

But by the time they returned Monday morning, it had been transformed into a child's fairyland, with mazes and swings and castles and spaceships.

All it took was four days--and the help of almost 700 volunteers. The result: the only playground in Fairfax County designed specifically for handicapped youngsters.

It was the closest thing to an old-time barn raising ever seen by most of those who took part.

"It's amazing," said Sarah Johnson, leader of the volunteers from the junior high youth group of Emmanuel Church on the Hill. "Four days ago it was nothing."

It was all for a good cause, said Joan Jason, one of 20 teachers at the school, which serves primarily physically handicapped youngsters from the eastern half of the county. "We wanted a project that would focus on the children," she said.

"There was no place where a parent could take a handicapped child to play with other children," added Gayle Burgoyne, another teacher at Key School, located at 6404 Franconia Rd., Springfield. "Usually one child would sit in a wheelchair and watch while his brother or sister played on a swing."

The chaotic construction of the tiered playground was the culmination of a yearlong project started by the school's Parent Teacher Association.

The PTA chose an expert to design the playground: Robert Leathers, a New York architect who has designed 80 playgrounds on the East Coast. Leathers in turn went straight to his own experts for advice: the youngsters at Key Center.

"One child wanted a castle," recounted Linda Culhane, whose 11-year-old son attends the school. "Another one said, 'I want a witch.' He paused, then he reconsidered, 'No, maybe I don't really want a witch.' "

But they got the castle: a maze of cubicles perfect for climbing with a design ripe for conjuring up images of dragons and princesses.

"Leathers' philosophy is that he doesn't want to stunt a child's imagination," said teacher Bob Read. "You want to keep the design flexible enough to let the child be creative and imagine things."

The playground gives handicapped youngsters an opportunity to do many of the things they have only been able to watch friends do, say those involved in its construction. There are special swings and ramps for wheelchair-bound youngsters and tunnels for children who can't walk. There are cubicles covered with textured materials for blind students.

The playground also is designed to challenge children who aren't handicapped. It will be open to all county residents after school hours and on weekends, project coordinators said.

"Designing a playground for the handicapped requires a lot more planning," said Leathers, adding that the Key Center project is the largest such playground he has designed. "You have to provide a broader range of activities."

Parents and teachers at Key Center who organized the effort can look back on it with pride, but they remember that they spent as much time panicking over the project as they spent praising its progress.

On Friday, for instance, when the Air Force promised to send 50 volunteers for the morning, 120 servicemen showed up.

"No hammers--they had no hammers and all we had for them to do was hammer," said Burgoyne. "We needed 120 hammers and we needed them in 15 minutes."

Organizers scoured the neighborhood soliciting hammers and dashed back to the school with dozens of the tools, said Burgoyne. But that solved only part of the problem. The Air Force honor guard members -- who had blocked all their ceremonial obligations for Friday -- were so enthusiastic about the work that they stayed all afternoon. When dinner time arrived, "we didn't have enough food," said Burgoyne.

More volunteers to the rescue. They telephoned the restaurants that had donated provisions and quickly rounded up enough to feed the Air Force troops.

Some of the volunteer construction workers kept coming back.

"I was having fun," said honor guard member Bill Williams, who returned to the project on his off-duty hours Saturday and Sunday.

"It means a lot to us that some of them came back on their own," said Fred Vogel, whose 9-year-old daughter attends Key Center.

Volunteers at the site were quick to show off "their" rail they had just completed or "their" deck or "their" swing.

"I helped carry the pyramids," said Kathy Callahan, who spent Saturday morning at the playground with her husband, Skip, a member of the Shriners.

School officials estimate it would have cost at least $120,000 to build the playground commercially. With about 100 businesses and organizations donating materials, money and manpower, the project -- which received no county money -- cost about $25,000.

"The Board of Supervisors would never fund something like this," said board Chairman John F. Herrity, who helped the school solicit donations and volunteers from local businesses, because it is not the sort of thing considered necessary to the education process.

Because the school is so small -- 198 students this year -- Principal Jacqueline Nunn said there weren't enough parents to do the job. So they began recruiting neighbors, church groups, businesses and civic organizations.

The result was long, eager lines at the check-in table.

Teacher Jim White was coordinating the work crews, dividing the housewives and business executives and carpenters into two groups: skilled and unskilled laborers.

"You're considered a skilled worker if you can hold a circular saw and saw along a line," White shouted to the group.

Leathers, a human tornado on the work site, barked orders to each group and directed the nailing of each rail and the boring of each hole. With a work force of amateurs mistakes were bound to happen.

"Someone nailed up the wrong pieces here," Leathers said, "but we'll accept it anyway."

The youngsters at Key Center are bound to agree.