This story is part of a series on gentrification, which appears online and in the February 2015 print issue.

There’s not a lot of space. Hannah Kinmonth-Schultz lives in a one-bedroom apartment in Seattle’s Central District with her husband, their 3-year-old daughter and a dog that’s still bigger than the kid. The family couldn’t afford a two-bedroom place, but Kinmonth-Schultz says proximity to jobs and amenities in the neighborhood more than make up for living in cramped quarters. Anyway, it’s a step up from the studio they’d all been living in. “We did upsize,” she says. “The one-bedroom is a lot bigger.”

That’s a choice not all families are willing to make. Seattle has many attractive qualities, including a robust job market, with all the beauty of Puget Sound close at hand and Mount Rainier looming in the distance (at least on clear days). In fact, Seattle is now the nation’s fastest-growing big city, having surpassed Boston in total population in 2013 and likely soon to break into the top 20 for the first time in more than 50 years. But it’s not necessarily a place that is welcoming to families. It’s long been short on kids, with the second-lowest number of households with children in the country. As is true in other big cities, more families are moving downtown, but many of them aren’t staying. “Once their children reach 5, about half of them have left,” says Jon Scholes, president of the Downtown Seattle Association, who is himself raising a couple of small children downtown.

The city is doing a better job of listening to the concerns of downtown families, he says. Seattle is building a billion-dollar park along its waterfront, but it wasn’t going to include a children’s play area until families complained. Now various city agencies consult with parent groups regularly. Nevertheless, the problem of attracting and retaining families could get worse before it gets better. In Seattle, even before you get to the paramount parental question of schools, the question of housing looms large for families. There’s just not enough of it to go around. There’s lots of new construction, but it’s coming at the expense of single-family homes, which until recently were a Seattle staple. It might be great for a certain stage of life, but most people don’t want to raise their kids in microapartments -- sometimes called “apodments” in Seattle -- above cocktail bars or e-cig lounges. “It keeps dollars per square foot up, so it makes perfect financial sense,” says Tyler McKenzie, president of the Seattle King County Association of Realtors, “but it’s going to drive families away.”

Market forces, in other words, are threatening to make it permanently harder for families to settle into center cities. Major cities have become more desirable places to live, but they are still not suited for everyone. The downtown amenities that attract young professionals and empty nesters -- the bars, restaurants and nightlife options -- aren’t what families need. The result is that Americans, already used to segregation by income and race, are seeing another type of geographic separation, with people living apart according to their stages of life.

Some of the cities doing the best at attracting professionals in their 20s and 30s are also shedding families. In Washington, D.C., for example, the number of people ages 18 to 34 grew significantly in the first decade of this century. That helped bring the millennials’ share of the district’s population much higher than the nation’s as a whole, but at the same time the city was losing about 14,000 children under the age of 18. In Atlanta during the same period, the number of children declined by a similar number, representing a loss of nearly 12.5 percent of those under 18 -- with that rate twice as high, at nearly 25 percent, in Atlanta’s downtown and midtown. San Francisco, which has arguably the boomiest economy in the country, also has the smallest per capita population of children of any large city. Indeed, singles now make up a majority of households in all but a handful of the nation’s 50 biggest cities.

MORE: View gentrification maps and data for Seattle

No mayor or city official would ever come out and say, like some high-end maître d’, that children are not welcome. But the fact is that a lack of kids isn’t such a bad deal for a prospering city. Childless singles and couples pay their taxes without demanding much in the way of services, at least in terms of schools and playgrounds. “You’re going to get that revenue, whether people have kids or not,” says Jacob Vigdor, a public affairs professor at the University of Washington in Seattle. “If your families don’t have kids, then your demand for local services is going to be really low. That makes it easy to balance your budget.”

Conversely, with families forced out of the city proper, that can create new challenges for outlying neighborhoods. As is true elsewhere, poverty is becoming more concentrated in Seattle’s suburbs. But the middle-class families with children moving to suburbs throughout the region are also putting more pressure on roads and public safety. “For us, just having families live here and commute up to Seattle has its negative aspects,” says Pierce County Executive Pat McCarthy. “On the one hand, we welcome young families and they build good communities. On the other hand, we need the economic development that keeps a community thriving.”

All of those trends worry Seattle Mayor Ed Murray. Creating a monoculture, with just one or two slices of the population able to live in the city itself, does not make for a bright future, he says. Murray recalls that when he was growing up, his working-class parents were able to afford a place big enough to house seven kids. That’s no longer an option. “Are people who are working but not making enough to buy or rent here, are they going to be shut out forever?” he asks. It’s been great for the city to be generating tech jobs that are filled by talented young people, but he wants the city’s fishing fleet and manufacturing sector to remain viable as well, offering outlets for residents at all stages of life. “If cities like Seattle don’t get this right, then what you see today as great is not sustainable.”

To deal with such concerns, Murray has created a task force to address housing affordability and livability. With so much growth, everyone sees the need for greater density and more units. But for families, the issue is the kind of units being built. There are lots of infill units going up around town, and living in someone else’s backyard may become the 21st-century starter home. But far more apartments are being built, most of them without families in mind. According to the city’s planning department, only 2 percent of Seattle’s market-rate apartment units have three or more bedrooms. Most of those are not readily affordable.

Canadian cities such as Toronto and Vancouver have enacted requirements that new condo and apartment buildings include certain percentages of two- or three-bedroom apartments. With Vancouver so close, it is often cited as an example of what Seattle could do. Still, regulations of this type are not likely to take hold anytime soon as they are seen as too strict. For now, new housing starts are mostly appealing to professionals with minimal family obligations. “With the way development patterns are going right now, it’s all geared to the 20- to 28-year-olds who are living by themselves,” says Bradley Calvert, a downtown Seattle resident who runs a website called Family Friendly Cities. “You’re not seeing any other kinds of development.”

Everyone understands that the city is going to grow denser and more crowded. Amazon is creating 16,000 new jobs in the city. Vulcan, the umbrella company owned by Microsoft co-founder Paul Allen, is also undergoing a massive expansion. Those two companies alone are building 10 million square feet of office space by Lake Union, at the northern end of downtown. Even traditionally suburban companies such as Russell Investments and Weyerhaeuser have each moved hundreds of workers downtown. All those people need places to live, so new apartment buildings keep sprouting up. Construction cranes downtown signal that more high-rises are on the way, and it’s hard to find a neighborhood anywhere in the city where at least a few five-story apartment buildings aren’t freshly built or still going up. The residences inside them are hardly spacious, even for just two people. And given that Seattle codes allow up to eight unrelated people to live together, there’s no guarantee that bigger units would be occupied by traditional mom-dad-kid families anyway. “You could get eight Amazon employees or eight university students,” says Nathan Torgelson, deputy director of the city’s planning department.

Convincing more parents to raise children in apartments, as has long been common in some Eastern cities, will require an enormous change in mindset, says Sally Clark, who chairs the Seattle City Council’s housing committee, especially for families with more than one kid. But even those willing to make that shift may not have the option. Clark says newly constructed two-bedroom apartments that aren’t publicly subsidized are like “unicorns.” She knows some people are worried that Seattle’s apodments could go out of style and end up devolving into modern-day fleabag hotels. “I don’t think we’ve yet seen the house that middle-class families can afford, if they’re not in the higher-income range of the tech companies,” Clark says.

The city has struggled to provide affordable housing for families, off and on, for years. But the decline in Seattle’s share of children has been in the making for an even longer time. The city was home to nearly twice as many kids back in the 1960s as it is today. Many white families began moving out of the city in reaction to forced busing and other factors. White flight seemed to level off, but well into the 1980s about a third of families with children moved out of the city once their kids reached school age.

Today, with soaring prices on homes near good schools that offer spare rooms or enough storage for kids to indulge, say, an addiction to Legos, Clark understands why residents are heading out to suburbs such as Auburn and Kent, where they can catch commuter rail lines back into the city for jobs. After all, families have been seeking more space out in the suburbs for decades. But Clark wishes Seattle were offering them more ways to stay. “I would love to keep more of those folks from thinking that’s their only option,” she says. “If you chose that because you couldn’t find anything in Seattle -- and I think that’s [the point we’ve reached] -- that’s sad to me. We’ve seen lots of people leave the city.”

You can walk the trail around Green Lake, not far north of downtown, and see plenty of joggers and bikers and friends strolling along in pairs, but not spot a single child. The same is true in Fremont, a section of town where the values suggested by an oversized statue of Lenin are belied by the presence of many upscale boutiques. There are no kids in sight -- not even in a bakery called Simply Desserts or a gelato shop that recently offered happy hour discounts in honor of its own 10th anniversary, just after school hours.

Those parts of Seattle that do seem to have a lot of kids are home only to about as many children per capita as the national average. Nationwide, 1 in 3 households includes a child under 18 years old; in Seattle, the number is less than 1 in 5. Even if you could solve the city’s housing problem, schools in Seattle, as is the case in seemingly all big cities, remain a big issue. The district has lately been changing superintendents on an annual basis. One of its major challenges, says Mayor Murray, is that while a majority of the city’s school-age population is white, a majority of the students in its public schools are not. Over the past few years, public-school enrollment has shot up more than 10 percent. But much of that growth came during the recession, when some families found themselves unable to move out of the city. Many of the new students come from lower-income immigrant families. “It gives the school system a bad rap because of the challenges presented by underserved communities,” says Peter Steinbrueck, a former member of the city council.

Right now, some Seattle schools are overcrowded, with several thousand of the city’s children sitting in portable classrooms. As a result, the district is taking back control of buildings it had leased out as adult education and community centers, and even a Nordic heritage museum. But many of its facilities remain underutilized. In 2007, the U.S. Supreme Court ended the district’s practice of assigning students to school by race. That has had the effect of making some schools north of the shipping canal that links Puget Sound and Lake Washington more white and the real estate around those schools more expensive. “A family with the median income in Seattle” -- $52,000 in 2011 -- “cannot afford to purchase a standard three-bedroom, two-bath home in any of our neighborhoods with high-performing schools,” says McKenzie of the Seattle King County Association of Realtors. “The middle class is being priced out of our marketplace because of our rapid growth and lack of available inventory and land that can be developed.”

Overall school enrollment, meanwhile, remains just barely over half of what it was 50 years ago. Families that can afford to do so are voting with their feet -- if not out of the city, then out of the district. Elementary schools may be more crowded, but attendance has stayed mostly flat for junior high and high schools. Becky Shull, who works in sales in the health-care field, lives in a neighborhood where she’s been happy to send her two children to the elementary school, but she’s not likely to keep them in the system. “The middle schools are kind of failing, in my opinion,” she says. So she’s looking at private schools. “That is an expense that we hadn’t counted on when we started raising kids, but it’s just part of the deal.”

Shull can afford it. But like other parents raising their children in Seattle, Shull says there are benefits she wouldn’t soon give up. Her family takes full advantage of the theaters, the museums and restaurants with “cutting-edge chefs” -- amenities she wouldn’t want to turn into weekend-only treats. Her quick commute not only saves time and money spent on gas, but is better for the environment. And her children get to experience a varied life at a walking pace, rather than seeing the world through tinted glass at 35 miles per hour. She also says she doesn’t care about not having a big yard or an attached garage.

But she recognizes that there are upsides to raising kids in the suburbs. Aside from the question of schools, there are all kinds of hidden costs for parents living in the city, including longer waiting lists for pricey child care and summer camps. Not to mention the pain of trying to find parking when you go grocery shopping or run other errands. “I feel like if you move to the suburbs, you don’t come back,” Shull says. “When you downshift, you don’t shift back up.”

This story is part of a series on gentrification, which appears online and in the February 2015 print issue.

*Correction: A previous version of this story incorrectly stated that a 2007 state Supreme Court ruling ended the district’s practice of assigning students to school by race. It was a U.S. Supreme Court ruling.