In the basement of a house furnishing day care to children of working mothers, a lone attendant was in charge of 16 pre-schoolers. When fire broke out, a half-dozen died.

In another day care facility, an infant sleeping on a couch because no crib was available was struck in the head and killed when an older child dropped or swung a heavy toy.

Such incidents, described by an official of the Child Welfare League, are unusual. But they illustrate in the extreme some of the problems arising from a mushrooming need for day care brought on by the remarkable growth in the number of working mothers, many with low incomes.

After letting the issues simmer on a back burner for a few years, Congress must face them soon.

President Carter's welfare bill, as amended by a House subcommittee, sets aside a minimum $400 million in the jobs section to provide day care for children of low-income working mothers. Some experts think minimum standards for the quality of such care should be written into the bill.

The Department of Health, Education and Welfare has sought to impose minimum staffing ratios for day-care centers receiving federal funding. The current version would require one staff person for every four children aged 6 weeks through 2 years, one for every five children aged 3, one for every seven children aged 4 and 5, and one for every 15 or 20 older children.

However, operators of private centers, as well as many members of Congress, said that these ratios would raise the costs enormously of both public and private day care, with at best a marginal and unproven gain in safety and child development. Many women, it was argued, would be priced out of the market. As a result, Congress has suspended the ratios for children under 6. It must decide by October whether to continue the suspension or put the standards into effect.

HEW, meanwhile, is studying whether these specific standards are in fact justified.

Seven years ago President Nixon vetoed a bill pumping an added $2 billion into day care. Now Sen. Alan Cranston (D-Calif.) is working up a new aid measure. He must decide how much to seek (it will be "substantial" he said in an interview), and what standards, if any, to impose on recipients.

Today, there are about 47 million children [WORD ILLEGIBLE] or younger in this country - and the mothers of nearly half of them are in the labor force. Even for children under 6, the proportion of working mothers is close to 40 percent.

Although most people envision day care being provided by centers away from home, such centers aren't what most people use. Estimates by HEW and the Census Bureau give this picture of the nation's 47 million young children.

Nearly three-quarters are cared for by a parent at home, primarily in families where the mother doesn't work but also where one parent works at night and the other during the day.

Another 4 million or so, largely children of working parent, are taken care of in their own homes by relatives or persons hired to watched them.

1.5 million or more, mainly school-age children of working parents, stay home alone when they return from school. "Perhaps this is no problem for a 13-year-old in a safe neighborhood," said an official of the Children's Defense Fund, "but what about an 8-year-old in a high-crime neighborhood?"

About 5 million go to the homes of other people who watch them for a fee. This "family day care" can range from fairly elaborate establishments to informal arrangements ("Will care for your child in my home $5 a day.")

About 1.8 million at most spend 10 hours or more a week in formal day care centers. This figure includes up to 400,000 in Head Start programs and some nurseries (but not elementary schools or kindergartens).

William Prosser, chairman of a federal interagency task force, estimated that total public and private spending on day care (excluding in-home care by parents, of course) is about $10 billion, with about $6 billion coming from parents, $2 billion to $2.5 billion in federal money, the rest from state and local governments.

Of the nation's 20,000 formally licensed day care centers, nearly half are run by private organizations for profit, others by Head Start or community organizations nonprofit and often free.

of the 1.8 million children enrolled full or part-time, not all have mothers who work. Some parents simply want a good nursery.

Conditions in the centers vary widely. According to HEW, most are licensed under state law. About two-thirds of those receiving U.S. funding or caring for federally aided children meet the suspended minimum federal staffing standards.

The average cost per child in such centers is about $175 a month in nonprofits and $125 in for-profits. In commercial centers that don't get any federal money and therefore aren't subject to federal regulation, the average monthly cost is $105. The profit centers generally have a lower ratio of staff to children.

Perhaps the most crucial issue in day care is the trade-off between costs and standards. It is often much cheaper for a working mother to send her child to a neighbor's "family day care" business than to a formal day care center. It is sometimes more convenient as well.

But most family day care arrangements are unregulated. The child might receive excellent care with loving attention in a safe atmosphere. Or, according to spokesmen for the Children's Defense Fund and the Child Welfare League, the setting might be a firetrap with a cranky person in charge who does nothing with the children and simply keeps the TV on all day.

To enforce minimum standards and training on all such situations probably would take billions plus an army of inspectors, to the point where many low-income persons simply couldn't afford it - nor could the government.

Perhpas the most envenomed argument is whether day care centers and other regulated places should provide an enriched, educationally stimulative program like Head Start, which employs reading-readiness, math-readiness, educational toys and at least some persons with training and experience in early-childhood education. The desire to provide at least some thing of this sort - if not as intense as Head Start - lay behind the staff ratios developed by HEW for day care centers.

Head Start was devised for poverty children, severely deprived both economically and educationally. Many studies and experts assert that Head Start-type programs, with high ratios of staff to children, do indeed produce long-term cognitive and developmental gains.

Allen Smith, director of special day care studies for HEW, said a new study by Abt Associates has demonstrated that keeping children in small groups of under 15 with perhaps a couple of teachers has more developmental impact than adhering precisely to a one-to-four or one-to five ratio, especially if the caregiver has some training or experience.

Smith said that precisely how much gain results from adhering to strict ratios is as yet undetermined.What is clear is that putting the suspended ratios into effect would cost a lot more. It would be at least $35 million for centers subject to the federal regulations, and substantially more if the states then made all day care subject to the same ratios.

The Children's Defense Fund and similar groups take Head Start as their model and favor a big boost in federal outlays for child care, enlargement of facilities, upgrading of standards at day care centers and family day care home, and aherence to the proposed staff ratios at least for pre-school children.

Others are not so sure that these extra billions would return a big payoff to society in increased achievement. A recent General Accounting Office study asserted, "The developmental needs of most children could be met with standards requiring less staff."

A House welfare expert, who asked not to be named, said. "A lot of people feel the case for the federal ratios is not established. Lots of people may be ederal ratios is not established. Lots of people may be poor and not well educated but they have a pretty good feel for what's good for their children. They take a good look at these family care homes to see what's going on."

Other s argue that only the most educationally deprived children need high-cost inputs.

Secretary Joseph A. Califano Jr. has announced that HEW won't come up with new staffing and standards recommendations for at least a year.

Meantime, the last word may belong to Senate Finance Committee Chairman Russell B. Long (D-La.).

"There is no evidence for the federal minimum numbers. No evidence except a desire for an enlarged federal bureaucratic structure," Long declared in an interview. He said the minimum staffing numbers should simply be suspended again for another year, leaving centers to adhere to less stringent state requirements.