* Neighborhood schools over 90% of people walk to. Builds community.
* Lots of parks, both near the schools and elsewhere. Gives kids a place they can go by themselves and have some independence.
* Dense neighborhoods so it's easier to go play with your friends rather than play video games over the internet (they still do the latter sometimes of course)
* Nearby shops. Kids and parents can walk to the grocery store, movie theater, bookstore, pharmacy (ie candy store for the kids)
* Wide sidewalks and relatively safe streets for kids to walk to these things.
* Bonus: we've got a trolley that stretches the neighborhood and goes into downtown Boston for the older kids.
You can find pockets like this (sans the train) across the US, but they're just that: pockets. I don't know of any city or town that has all these traits in its entirety (South Brookline for example doesn't cut it, for example). However, as long as they're scarce they'll be expensive. I hope we see a move towards building more neighborhoods like it so that more kids can grow up in them.