Last year, a report from City Observatory titled “Young and Restless" found a fascinating trend in Atlanta. Between 2000 and 2012, there was a huge boost in the number of young, educated adults in the center of the city — an increase of 38.7%. This is significant for a city that has experienced a general population decline since the 1970s.Fostering an urban environment that suits this key demographic is a success worth trumpeting, but I wonder how long these Millennials will stick around. In the long run, what happens when many of those young and educated new residents want to have kids — will they move away from the city to find family-appropriate housing and good schools? How sad it would be to lose this gain in population because of our inability to provide a place for new families.[BTW, Thomas Wheatley at Creative Loafing has an excellent piece on this topic. “Stick around, Millennials" — you should go read it right away]What the Census tells us about families and housing in AtlantaA look at Census data makes the problem clear: though intown neighborhoods might be doing well with young singles, they seem to be in danger of creating child-free zones that are fairly incapable of attracting families with kids. Of the 61 largest cities in the US, Atlanta ranks among the lowest when it comes to our population’s percentage of kids under 18 — 19% of the population of the City of Atlanta is made up of kids under the age of 18, well below the national average of 24%.Also, the number of kids under the age of 5 in the City of Atlanta declined 2000-10 while this same number boomed in the fringes of the metro suburbs.(No doubt, many people living in those child-rich zones on the fringes are commuting long distances to work by car to job centers near the city center, making the lack of family-friendly places intown a contributor to the region’s notorious car congestion.)And though we’re losing when it comes to kids, we’re on top with another count.  Atlanta has the highest percentage of single-occupancy dwellings of all large US cities. We’ve become a hot spot for singles, but the outer ‘burbs are still the big draw for families. School quality is no doubt a factor, but housing stock is also an issue.44% of dwellings in the state of Georgia have three bedrooms, while only 25% do in the City of Atlanta. This situation probably suits the needs of young adults and their current lifestyles, but what of their future? Our bedroom deficit puts the city far below average for a basic need when you look at common family sizes. If we want to undo the trend of families choosing suburbs over intown neighborhoods, one of the most important things we’ll need to do is provide places for them to live.Losing out to other cities and suburbsLooking over our shoulder at competing cities is a major pastime of Atlantans. Take a look at this fact: Seattle, Pittsburgh and others are making gains in drawing families to their city centers. Will we lose out to those places when Atlanta Millennials look around for places to raise their kids?When my own family was on a house hunt a few years ago, local real estate agents seemed confused by our search for a condo (worst of all — a gasp rental) for us and our child. They gingerly suggested we look into a suburban house as a better option.Families shouldn’t feel that they have no choice but to live in a cul-de-sac subdivision outside the city.I believe that the quality of a city’s design can be judged by how well it treats its neediest, most vulnerable citizens — and children are included in that group.Housing, livability, and the great diversity of a healthy neighborhoodMaking cities livable for all groups is essential to their success. For true sustainability, cities need to be welcoming to a diversity of income groups, ages and family sizes. If it’s true that a growing number of today’s single urbanites will want to remain in the city center after they have kids, some work needs to be done to make them all welcome. We’ll need more transit-connected housing with multiple bedrooms, better schools and more parks.Walkable, compact urban places are not the exclusive domain of young creative people. They’re great for everyone; singles, couples, families with children, retirees — and all income levels. It’s important to have a good mix of residents from all these groups in a neighborhood because they each bring strengths that benefit not only individual neighborhoods, but overall cities.Good urbanism requires a good mix of residents: renters who allow new developments nearby without shouting “Not in My Back Yard”; owners who keep an eye on long-term livability; singles who are happy to see the new nightlife spots; and families with kids who push for a new park with a playground.With that kind of diversity, you end up with a healthy neighborhood and a healthy city.