Some do it to seek an urban lifestyle, shorten commutes or be within proximity to restaurants, museums and other attractions.
Along the way, they're pushing cities to be more welcoming to families. Parents in cities across the country, including Seattle, Minneapolis and Denver, are banding together to advocate for new downtown schools, more playgrounds and more affordable family-friendly housing.
"The benefit of living in the city so far outweighed what we would get by moving out to the suburbs," said Jenny Kelly, 32, a marketing consultant who helped form a parents group in 2013, now called Parents for a Better Downtown Seattle.
She and her husband Michael moved downtown in their 20s and stayed when they had 2-year-old Elea. Both walk to work from their loft apartment in Pioneer Square, where the laundry room doubles as the toddler's bedroom.
Kelly and other parents recently pushed for a public school downtown, and they lobbied to include a playground in a redesign of the city's waterfront.
Seattle offers developers near downtown the chance to build taller if they provide space for a school, though no one has taken up the offer.
Some cities are trying to respond to families, albeit slowly. Portland revised building codes to allow courtyard housing, considered an amenity for families. And Baltimore Mayor Stephanie Rawlings-Blake has pledged to grow her city by 10,000 families.
"Families with children are indicator species about the health of an urban neighbourhood," said Gary Johnson, Seattle's city centreco-ordinator.
For decades, Seattle has had a well-earned reputation as a childless city. In the 1960s and 70s, as more families moved to the suburbs, their numbers in the city dropped.
Children still make up a much smaller percentage of Seattle's population, compared with the rest of the state or the U.S, but recent trends suggest Seattle is doing a better job of holding onto those kids, according to Sightline Institute, a Seattle-based think-tank .
The city is growing its share of kids under 15 at a time when that percentage is declining elsewhere.
"We seem to be closing the child gap," said Eric De Place, the group's policy director.
Between 2007 and 2012, the number of downtown kids enrolled in kindergarten to eighth grade jumped nearly 30 per cent, according to the Downtown Seattle Association, a business group that has worked to get more families downtown.
Parents used to stay until their kids reached age 5 and half would leave downtown, said Jon Scholes, the association's CEO who is raising 5-year-old twins downtown. But he's starting to see parents staying longer than before. Schools, parks and other amenities matter, he said.
In Minneapolis, Melissa and Aaron Whitney downsized from a 2,700-square foot five-bedroom home in the suburb to an apartment half that size in downtown.
"We drank the Kool-Aid. We thought once we got married, we had to buy a house in the suburb," said Aaron Whitney, 40, a technology consultant.
They quickly realized they didn't want to spend their time on home repairs or yard work and already spent so much leisure time downtown that they moved back with their son, who is now 2.
The couple said they wouldn't mind more green spaces and affordable housing in the city. A new school opens up in downtown this fall, and they're watching its progress to decide where they'll enrol their son.
Bradley Calvert, 33, who moved from Atlanta to Seattle with his wife and toddler last year, joined other parents in pushing for a downtown public school. The district last month lost a bid on a vacant federal building downtown.
He said a downtown school and more affordable housing will be keys to keeping families in the urban core. A city benefits when there's a diverse mix of people downtown, said Calvert. Attracting families, not just young professionals or empty-nesters, creates a sense of community.
Lindsey Carillo, 31, grew up in a suburb of Detroit but wanted to expose her kids to city living. "It gives them a different way of living," said the mother of two kids, who are 8 and 1. "I want my kids to have a cultured experience growing up."
Her family of four shares a two-bedroom high-rise apartment in downtown Seattle. Occasionally, her son wishes he had a backyard and could walk to school, but "he loves to show off where we live as well," she said.
Paul Hughes and his wife Heidi didn't want to give up on urban living when they had a son.
The couple likes that their second-grader son, Silas, is exposed to more experiences in the city. They would like the city to do more, including adding playgrounds and a school. And they wouldn't mind having more playmates for their son in their building, either.
"It's nowhere near as challenging as people think it is," Hughes said. "The more people see it working, the more they think it's possible."