Good, affordable day care is a necessity for most working parents, but for low-income families it is often the difference between working and welfare. And lately, like a job, it's been getting harder to find.

In 1981, the Reagan administration cut--by as much as 25 percent--all major social programs providing direct support to day care. The government also eliminated provisions that earmarked $200 million solely for day care.

The Agriculture Department's food subsidy program also was cut, so that day-care centers serving poor children receive funds for two meals and a snack instead of three meals and two snacks. Because those subsidized children tend to come from families where parents work long shifts and commute long distances, that loss was particularly mourned.

Since then states have scrambled--some with more success than others--to maintain their subsidized day-care programs.

The Children's Defense Fund estimates hundreds of thousands of children have been affected by the funding cuts, some losing a government funded day-care meal, others losing care altogether. Centers that served large numbers of low-income children have cut their staff, others have disappeared.

"Adding it all up, it's pretty bleak," said the fund's Helen Blank. "If more and more of this country's fortunate children are in preschool and day care and poor children don't have that opportunity, we're going to see greater and greater disparity between the two."

To cope with the diminished resources, Virginia turned its day-care funding problems over to individual counties, and while some centers suffered, those in Northern Virginia held up well. Fairfax County, for example, managed to preserve every subsidized slot the federal government dropped.

At the same time it is cutting social service funds, the Reagan administration is expanding the tax credits available for child care, which cost the Treasury $1.2 billion this year.

While day-care tax credits are available to all parents, it is the middle class that makes most use of them, day-care specialists say. This year, after lobbying by child advocates, the administration agreed to include the tax credit deduction on short tax forms, making it more accessible to low-income families.

Children's advocates say the federal budget cuts have inhibited the ability of state officials to monitor day-care center regulations and have led to a general erosion in day-care standards. Department of Health and Human Service officials say they have not noticed much slippage but add they do not monitor states' activity as much as they once did.

Advocacy groups also are concerned by the four-year effort of church groups around the nation to have church-sponsored day care exempted from state licensing.

More than seven states, including Virginia, have passed laws that exempt church-run day-care centers from licensing and in some cases health and safety requirements. Other states have laws pending.

Child advocacy groups say they believe the scattered but bitterly fought legal battles over exemption are part of a national effort on the part of conservative Christian churches.

"They are trying to get to a point where no one has to look at what they're doing with children except parents," said Joe Perreault of the Save the Children Foundation. "In a large center situation, parents can be easily fooled into thinking a child is getting more supervision than he is."

Rules for family day-care homes, where a woman takes children into her home, also are changing. Family day-care homes are a mammoth and largely invisible cottage industry estimated to care for about 25 percent of all children in day care. Most states require licensing of the homes, but only about 10 percent of those in operation are believed to be licensed.

Now, the Department of Health and Human Services reports, about 15 states have switched from licensing to registration, on the theory that fear of not meeting state standards is what keeps home operators from coming forward. In some states, registration brings homes under almost equal state scrutiny but in others registering means little more than signing a form and paying a fee.