Summer vacation yawns bright, hot and endless in Fairfax County, made to order for a latter-day Huck Finn. Asphalt melts, swimming pools shimmer and there are creeks, 7-Elevens and Pac-Men to be discovered.

Unless, of course, you are one of the 12,000 county elementary school children who deserts cul de sac and subdivision six weeks each summer and heads back to school. On the theory that busy is better, their parents have paid the county $10 to enroll them in one of 117 "neighborhood centers," where for seven hours a day, five days a week, "organized recreation" reigns.

It is cheaper than summer camp, frequently enjoyable, and for parents the best thing to hit suburbia since the sprinkler. For the kids, it's as sure a sign of summer as Little League. It's not just the bubble-gum blowing, the ice-cream making or the papier-mache classes that bring them day after day, though that helps. Instead, the centers are a six-week escape from the isolation of summer in suburbia, where highways separate school chums, a driver's license is the ticket to happiness and, more often than not these days, mother works outside the home.

"It's a life saver for me," says the mother of Damien Bravo, a 9-year-old attending the center at Mosby Woods Elementary School north of Fairfax City. "And he's an only child, so it's very good for him."

"It's okay," says Damien. "I like it. If I wasn't here, I'd probably be home watching T.V."

"And eating," adds his mother.

The rules at Mosby Woods are simple and prominently displayed: No hitting. No biting or kicking. No pushing. No games during quiet time, no wandering off. "If it's not yours, leave it alone." And, finally, "have fun."

They do. Though it may sound dreary to adults who spent summers at the swimming hole, the children at Mosby Woods are modestly enthusiastic about their organized vacations.

"I'd rather be here," says 7-year-old Duncan Holley, one of 90 children who has signed on for the summer hitch. "At home I'd probably just be listening to tapes in my room."

"They gave me a choice," says Tana Rhodes, 11. "Either come here or go to the pool. This is better, most of the time."

In the last three years, says Ray Graham, director of the county's summer recreation program, he has answered innumerable calls from working mothers with a houseful of summer scholars to provide for. At Mosby Woods, most of the children cite working parents as their reason for attending: "My mother wants us to come here because she works at the school," or "My mother is a typist," or "My mom visits prisoners at the jail," or "She works at Sears."

"My Mom works and my Dad works," says Duncan. "I think if we stayed home I would play kickball with my dad. My Mom doesn't know how, so we could teach her, I guess."

"We're an alternative," says Graham. "Some of the parents think of us as a babysitting service. We're not, but as long as they bring the children, I don't care what they call us."

Fairfax County has operated its neighborhood centers for more than 20 years and will spend almost $350,000 this year on the program. Graham has long endured criticism from people who say "organized recreation" sounds too much like boot camp. "Anyone who says that hasn't been a child in some time, and certainly hasn't had one."

Mosby's raiders arrive at 8:30 a.m. and leave after 3 p.m. By midmorning on a recent day, the 8-to-13-year-olds have forsaken games on the baking blacktop for the auditorium's steamy gloom. The younger children, aged 4 to 7, sprawl like hounds on the cool linoleum and listen to the children's story of "Madeline," a Parisian girl at a convent school.

"What's this book about?" a teachers' aide asks when the last page has been turned. The only thing anyone remembers is something about a dog. So much for French literature.

Lunch is soon announced. Under the gaze of Tammy Harrison and Janet Williams, two Fairfax residents and Brigham Young University education majors, and Christie Milburn, a 14-year-old Fairfax CETA worker, several dozen peanut butter and jelly sandwiches are quickly removed from battered lunchboxes and consumed. Then it is time for games.

The only remotely sophisticated equipment in the room is an old phonograph. The children don't seem to mind this. And the parents, whose common prayer seems to be "Deliver them from video," think its grand. "Anything to keep them from playing Pac-Man or Atari 20 hours a day," says Winifred Johnson, mother of a second-grader.