Prince William County nearly canceled its school-age child care program last week because county officials had unrealistic expectations that the venture would be instantly popular and immediately pay for itself, politicians and day-care officials said.

The Board of County Supervisors last week voted 5 to 2 to spend $100,000 more than expected on the program and offer less than was planned. Initially, there was to have been a child-care program at seven elementary schools, but that was cut to five because children had filled fewer than half of the 420 slots the program would have offered.

The program will begin as scheduled in September, but the county will have to subsidize those schools where fewer than 27 children sign up for each session.

"The expectations were too high," said Wynettia Slaughter, who runs Manassas Park's day-care program. She noted that when Fairfax County's popular program was started, there were vacancies at all day-care centers.

New programs rarely attract the kind of attendance Prince William officials expected and didn't get. Manassas Park filled only 15 of its 36 slots in its first year, and Chesterfield County, a Richmond suburb, drew only 150 participants in its first year, even though a survey identified 7,000 "latchkey" children in that area.

According to county staff members, the seven original sites were selected for political expediency -- one in each magisterial district -- and because the principals were interested, rather than on proven need. The citizens committee that helped design the program had less than six months to do it, and applications were sent to 4,200 eligible families in July and were due back in early August.

The organizers also drove away some parents by closing the program on five teacher work days, when students do not attend class.

"I've had my children in my office because I was unwilling to leave them at home alone. A lot of people don't have that luxury," said Health Director Jared E. Florance.

The debate over providing day care for school-age children has flourished in Prince William -- the only major Washington area jurisdiction that doesn't provide such care -- since a 1986 Health Department survey found that nearly 20 percent of parents left their elementary school-age children alone or with siblings.

The struggle pitted parents and teachers concerned about unsupervised children against fiscal conservatives reluctant to subsidize a program in competition with private day-care centers.

County officials tried to appeal to both sides by contracting with a private firm, Minnieland Private Day Care, to run a program that would cover its operating costs by charging parents $48 a week for both morning and afternoon care.

That goal was unrealistic, day-care experts said. Few county-sponsored day-care centers break even. In Fairfax, where the 15-year-old program supervises 5,000 children in 101 centers, fees cover only $6.2 million of the $8.9 million cost, said spokeswoman Patricia McClenic.

Subsidies often are higher for newer programs. Manassas Park's two-year-old program required a 50 percent subsidy last year, Slaughter said.

The county's insistence on minimizing subsidies undercut the program's efforts to attract parents who left their children alone. Most parents who have signed up for the county's program are switching their children from private day care, coordinator Karen Severn said.

"If they're leaving their kids alone at home, why should they pay to leave them at school?" said Patricia Dulaney, president of the Parents and Teachers group at Neabsco Elementary School, which will house a day-care program.

Both supervisors who voted against launching the program this year argued that the latchkey child problem may have been overstated and that the county should not subsidize a program that competes with the private sector. The supervisors represent the two school districts, Nokesville and John Pattie, where the day-care program has been canceled because of low enrollment.

"We said the program would take latchkey children, and we're not getting at that type of problem. I don't think the {program's} purpose is being accomplished," said William J. Becker (R-Nokesville).

Edwin C. King (D-Dumfries) argued that the private sector has stepped in to provide day care. "You're in competition with the businessman," he said. The number of county-certified day-care homes -- only one of several licensing categories -- has risen from 80 to 140 in three years.

Supporters countered that day care is badly needed in the county, where more than 55 percent of all workers commute to jobs outside the county. They said organizational problems and parents' natural reluctance to try an untested program explain the low enrollment.

"It maybe a risk factor. Parents are unwilling to give up their slots in other places," said School Superintendent Edward L. Kelly.

Despite the shaky start, supporters praised the county's decision to go ahead with the program.

"It's a good program. If you gave up on it now, I don't think you'll get it back," said Stewart Christiano, who headed the citizens committee that helped design the program.