Prince William County supervisors decided yesterday to scale back and subsidize a pilot school-age child-care program because fewer than half the 420 slots it would have offered have been filled.

The supervisors voted 5 to 2 to begin the program next month in five schools rather than the planned seven and to spend as much as $100,000 more than the $276,000 that had been budgeted for start-up costs.

Under the action, parents will still pay $48 a week per child for both before- and after-school care, but the county will subsidize operations in schools that fail to reach the 27-child break-even point.

The county staff had recommended scrapping the entire program because demand was much lower than expected and Prince William is expecting financial difficulty caused by major state revenue shortfalls.

But Supervisor Kathleen K. Seefeldt (D-Occoquan), citing the four-year drive for day care, said, "If we don't get something started in September, we're going to have a serious credibility problem."

The Board of Supervisors also voted to ask voters to approve a $43 million bond referendum to build a new mid-county road. The referendum, which will be on the November ballot, provides money to pay for a four-lane alternative to overcrowded Davis Ford Road between Liberia and Hoadly roads. The project would also use $20.2 million from a bond referendum approved in 1988.

Opponents of the new road have charged that the county is misusing the money from the first referendum because the ballot question stated the bond was for improvements to the Davis Ford Road corridor.

The low response to the child-care program came as a surprise to county officials, because Prince William is among the few jurisdictions in the metropolitan area that do not provide child care.

Critics of the decision to provide child care charged that emotions had overridden common sense. They questioned whether the county's program would really help so-called "latchkey" children who are left unsupervised at home.

Most of the 108 families who have already signed up for the program are switching their children from private facilities for reasons of convenience or cost, said Deputy County Executive Lawrence A. Hughes.

"We're subsidizing something that is in competition with the businessman," said Supervisor Edwin C. King (D-Dumfries), who was opposed to starting the plan in September.

Officials said the program may also have been hurt by last-minute organizational efforts. The supervisors, under political and parental pressure, overruled a staff recommendation in December to delay the program until the 1991-1992 school year to allow more studies and better planning.

"We got our marching orders," said one staff member. "It was crash and burn."

Applications for the child-care slots were not sent out to the 4,176 eligible families until July. Only one site, Springwoods, came close to breaking even, and the two schools that have been dropped from the program -- Nokesville and John Pattie -- each had fewer than 10 children signed up.