Imagine you take a time machine trip to 2037. You step out and start to explore your city. What sights and sounds would convince you that the Philly of the future was thriving?

Here's what I would check for: children. Children playing outside. Children exploring on foot, by bicycle and public transit, getting to know and enjoy their streets, their parks, their local area, and the wider city. Children spreading their wings and taking those gradual steps to becoming active, engaged, responsible citizens. Learning all the time from their own efforts, and their own mistakes. Like we all did when we were young.

Children are an indicator species for cities. The visible presence of children and youth of different ages and backgrounds, with and without their parents, in numbers, is a sign of the health of human habitats. Just as the presence of salmon in a river is a sign of the health of that habitat.

By this measure, American cities today are anything but thriving. Children's everyday freedoms - whether it's walking home from school with friends, or heading out to the corner store for a slushy - have been in decline for decades.

It is parents who make the judgment calls about when to grant these freedoms. So it is tempting to put all the blame on parents for the shrinking of children's horizons.

Tempting, but wrong. When parents today look out from their front doors, they see a world that is, at best, uncaring about their children, and, at worst, hostile to them. And no wonder, thanks to relentless traffic growth, rundown parks and green spaces, and eyeball-grabbing scaremongering in both the mainstream and social media (even though violent crime in America has plummeted since the 1990s).

It doesn't have to be this way. In my work as an advocate for more child-friendly urban planning, I have been to many cities where things are very different. In Tokyo, 6-year-olds ride the subway downtown to school alongside the suited businessmen and women. In Switzerland, parents who do not let their kids travel to kindergarten - yes, kindergarten - on their own are looked down on.

Or take Bogotá, the once failing capital city of Colombia. Over two decades city leaders - most prominently Mayor Enrique Peñalosa, reelected in 2015 - have reversed the city's decline. Hundreds of miles of streets and sidewalks were reclaimed for pedestrians. Elegant new parks gave the city green lungs. The Transmilenio - the world's biggest urban busway scheme - and an ever-expanding network of cycleways offered millions of people safe, reliable, affordable alternatives to the car. And the touchstone and compass point of this remarkable reinvention has been child-friendly urban planning. If Bogotá can do it, any city can.

Who pays when today's inactive children grow up to be tomorrow's unhealthy adults? Anyone who pays taxes or health insurance - in other words, all of us.

Who loses when cities are so car-dominated that parents feel they have no option but to get in their cars too? Not just commuters, but anyone who cares about pollution.

What happens when families who have the option move out of the city and into the suburbs? They take their taxes and spending with them, and the city's economy starts to crumble.

In Rotterdam in the 1990s, family flight was undermining the city's future. So city leaders have spent tens of millions of dollars making inner urban neighborhoods cleaner, safer, and greener. They are now reaping the rewards of what they unapologetically call state-sponsored gentrification.

So on Wednesday, from 5-7:30 p.m., when I share my vision of the future for cities at the Trinity Center for Urban Life, 2122 Spruce St., Philadelphia, my message will be simple:

If we ask ourselves what the sustainable, successful, healthy city of the future looks like, the answer is that it looks like a child-friendly city.