Hillary Clinton has said that helping working parents would be one of her primary goals as president and, last week, she let the country know what she has in mind. If she’s president, Clinton said, she’d work to make sure no family has to spend more than 10 percent of its income on child care expenses.

That’s a big goal. And it’d come with a big price tag.

Earlier this year, when the Committee for a Responsible Federal Budget analyzed the campaign agenda of Sen. Bernie Sanders (I-Vt), it assumed that his own, less detailed call for universal child care would end up costing the government about $35 billion a year. When Make It Work, an advocacy group, put together proposal for universal child care that would include children up to age 12, it suggested that it would eventually require $100 billion a year in new money.

Neither projection was particularly precise and, as the vast gap between the two suggests, the ultimate cost of any program could vary dramatically depending on program details that Clinton has yet to provide. Would the program cover all children, or just younger ones? Would it take effect quickly or gradually? And so on.

Still, the figures convey a sense of scale. Fully realizing Clinton’s goal would likely require the federal government, and thus the taxpayers, to come up with tens of billions of new dollars every year. That’s real money, even by Washington standards -- less than what the federal government spends on Medicare’s prescription drug benefit, most likely, but still a vast increase over what the federal government currently spends on Head Start and other federal programs for young children.

So do the advocates for such a program flinch at such high costs? Hardly. They say such a substantial investment in early childhood is long overdue. And they have a lot of evidence on their side.

The American family changed and public policy hasn't

It starts with the science. Researchers have shown conclusively that the first few years of life are a critical period for brain development. Young children who grow up in nurturing, stimulating environments are more likely to develop the skills they need to succeed later in life -- like learning how to control their emotions, or staying focused long enough to solve a difficult problem. Young children who grow up without those supports are at much greater risk of emotional, intellectual and even physical problems that can cause them to struggle in school or the workplace, or, in the worst cases, to run afoul of the law.

HuffPost infographic by Alissa Scheller

These are just some of the reasons that society has a strong interest in making sure infants and toddlers get consistent, attentive care. And up through the 1960s, society basically had it easy, because it could rely on what economist Heather Boushey has called “America’s silent partner” to perform this vital work.

As Boushey explains in Finding Time, an important new book on the economics of work and family, for most of the 20th century, the majority of young children got care at home, usually from their mothers. Sometimes these moms also had jobs outside the home. Sometimes they did not. In either case, they had nearly all the responsibility for their families’ child-rearing. And they provided it for free, at least in the sense that nobody was writing them a paycheck.

Nowadays the majority of women have jobs outside the home. As a result, the Gross Domestic Product and family incomes are higher, while women are finally getting the same kinds of career options that men have always enjoyed. But very young children still have mouths that need feeding, diapers that need changing and brains that need stimulating. As Boushey, who has advised politicians including Hillary Clinton, told the Huffington Post, “The silent partner is gone, mostly, but the demand for her work remains.”

Historically, these sorts of profound changes in America’s social and economic fabric have prompted the federal government to act. And in 1971, just as this major transformation in family life was getting underway, Congress passed a bill to create a national child care program. But Richard Nixon vetoed it, arguing, among other things, that it would “commit the vast moral authority of the national government to the side of communal approaches to child rearing over against the family-centered approach.”

Child care costs too much -- and does too little

That was the last time child care figured prominently in the national political conversation. Today, families paying for child care usually do so with little or no help from the government -- and the bills can be staggering, as recent reports from ChildCareAware and the Economic Policy Institute have documented.

Infant care, which tends to be the most expensive, can run well over $10,000 a year in high-cost states. In Massachusetts, the most expensive state of all, the average cost for putting a baby into an accredited child care center is more than $17,062. That’s more than 15 percent of income for the typical Massachusetts married family, and nearly 63 percent -- yes, 63 percent -- of income for the typical single parent.

Even at these high prices, however, the actual care is frequently mediocre and sometimes worse than that. Child care is subject to loose regulation and even looser enforcement. In the worst instances it means kids are exposed to serious emotional and physical hazards. A recent investigation of Mississippi child care providers by the Hechinger Report, for example, turned up reports of alleged abuse and clear safety hazards, including a center where one woman was watching over 59 young children.

Mostly, though, low standards for child care mean that while parents are working, the majority of kids are simply not getting the kind of attention they should. In 2007, when the National Institute for Child Health and Human Development surveyed care for children under 3, it found that even among formal child care centers -- which tend to be more professional and organized than home-based providers -- the ratio of caregivers to children was routinely below standards that the American Academy of Pediatrics and American Public Health Association have recommended as adequate. (Legal standards vary by state.)

Child care policy on the cheap could be bad for the kids

If Clinton were to succeed, the federal government would finally step in and take the kind of action that Nixon rejected in the 1970s. But the advocates and experts working on child care proposals stress that they don’t want to subsidize poor care, which might be good for the economy but bad for the kids. (Something like that may be happening in Quebec, which has been running a sweeping child care subsidy program since the 1990s.) They want to make sure that children are actually getting the nurturing they need.

That would mean, first and foremost, giving working moms and dads more freedom to stay at home with children when they are in their first weeks and months of life -- by guaranteeing paid leave to all workers, as every other country in the developed world does. Then, for parents with slightly older children who want or need to work, it would mean setting high standards for out-of-home care, including higher pay for child care workers.

Yes, it’s expensive. So are public schools and if we want our nation to be competitive, that’s the way to think about it. Heather Boushey, economist

That last part helps explain why a plan like Clinton’s could get so pricey. Today, median pay for child care workers is just $9.77 per hour, which works out to an annual salary of $20,320 -- less than parking lot attendants make. At those salary levels, it’s difficult to attract the most talented caregivers and then keep them on the job for more than short periods of time. As Marcy Whitebook, director of the Center for the Study of Child Care Employment at the University of California-Berkeley, said, “The low pay endemic to early care and education undermines building a skilled and stable workforce, shortchanging the majority of children and families.”

Simply raising the minimum wage to $12 or $15 an hour, as Democrats and their allies have proposed, would obviously make a big difference. Clinton has proposed additional measures that could boost wages for some caregivers even more. But if child care centers end up boosting pay for their workers and hiring more staff in order to improve the ratio of caregivers to kids, they’ll have to raise their prices, too -- which is why providing enough federal assistance to cap child care costs at 10 percent of income, as Clinton as proposed, could get so expensive.

The taxpayers would ultimately have to shoulder that burden, one way or another. But they would get something for their money. "Yes, it’s expensive,” Boushey said. “So are public schools, and if we want our nation to be competitive, that’s the way to think about it." Julie Kashen, policy director at the Make It Work campaign, said early childhood spending is actually an investment: “We know, from study after study, that the first five years of a child’s life are critical to what happens later.”

This isn’t just a talking point. Research on early childhood suggests that the spending tends to pay off in the long run, since it means future workforces are more productive -- and less likely to require remedial education, public assistance or attention from the criminal justice system.

The crude version of this, which every advocate repeats, is that every $1 of spending on early childhood care yields $8 in future returns. That’s probably optimistic, in the sense that it assumes all children are getting exceptionally high-quality early childhood programs with exceptionally talented instructors. But there's lots of reason to think that spending more on early childhood now would yield larger returns later.

Americans like to say that nothing is more important than a child’s well-being. But U.S. public expenditures on early childhood are currently among the lowest in the developed world. Clinton wants to change that -- and it's easy to see why.