Fairfax County is one of the nation's genuine boom counties. It's loaded with good schools and thriving shopping malls, swimming and tennis clubs. And its revenues are being expanded every day by a rapidly growing industrial and population base. For a mere $150,000 you can buy a four-bedroom roost in this little bit of heaven, provided you shop very carefully and don't mind a long commute downtown.

As one might imagine, most of its residents fall into one of two subgroups: very wealthy people whose income flows from one source, the husband; and the rest of us. The rest of us are either single parents who work to support ourselves, our children and our mortgage or two-income parents who work to support ourselves, our children and our mortgage. In either case, this is not a county in which the women are working for pin money. In 1984, per capital income was $19,825.

In 1970, the labor force participation rate for women in Fairfax was 43.4 percent. It was 61.7 percent by 1980, and by 1984 it was 64 percent. That's nearly a 50 percent increase in working women in the county in 15 years. Women make up 46 percent of the county's 362,613 working residents. In 1980, the most recent year for which the following data was gathered, 66 percent of all women with children ages 6 to 17 were working, as were 49 percent of those with children under 6.

More than a decade ago, a group of women who were active in community organizations such as the League of Women Voters spotted the demographic trend and realized families would need child care. After the predictable struggle to get county officials to realize the community was going to face serious child care problems, the county supervisors commissioned a task force on children.

It recommended establishing a county Office for Children that would be the central point for operating extended day care programs in schools, referral services to day care providers, training programs for family day care providers, and subsidized child care for low-income families. The board subsequently budgeted funds for the Office for Children, which celebrated its 10th anniversary last week.

In many ways, the county has been on the cutting edge of providing child care.

Its School-Age Child Care program is one of the most highly regarded in the country. It offers care before school and after school in 52 centers and five special-needs centers serving 1,800 students throughout the county. That program started with eight centers and has grown manageably by about 175 students a year. The summer SACC program operates out of seven elementary schools and has a center for children with special needs. The office's child care referral service gets 1,700 calls from parents a month. It lists 500 family day care providers and 150 centers.

The county's Board of Supervisors recently voted to open up the SACC program -- which has a perennial waiting list -- to children enrolled in private and parochial schools. Preference, however, will be given to children in public elementary schools already in the program and their siblings. J. Hamilton Lambert, the county executive, aware that this new policy may cause very thorny problems, promptly set up a task force of people from various county government departments to find out what should be done to expand day care. Jack Herrity, the chairman of the Board of Supervisors, wants to concentrate now on getting private employers involved.

Lambert sees "cafeteria plans," in which employes can select employer-financed child care as an optional benefit, as one of the coming employment trends in the county. He points out that the employers who provide child care service to employes "can enhance their overall operations while not having to spend a great deal of money." An employer who sets up a day care center, "cuts down absenteeism, cuts down the anxiety when an employe has to leave to pick up a child in certain situations, and he can use it as a tax benefit. That's three benefits to the employer."

A rising birth rate, low unemployment, a mobile private sector and great numbers of working women all have put pressure on the public and private sectors to provide child care, says Lambert, and that will continue. "There's no reason to have Fairfax County be the only day care provider. The need for this service is such that as long as it can be monitored, the marriage of private and public can go forward and provide that service."

What county officials understand is that child care for the population is as critical as schools and transportation. And a county board dominated by Republicans and chaired by a conservative is committed to providing it.