When I went to college, all of my suburban friends hadn't done too much of any of that, because being in the suburbs before driving age requires a parent available to be your chauffeur. Anecdotally, they were also more prone to underage drinking before college, because they just couldn't go out and do much.
That being said, at an older age, I never had to take my children anywhere. Like you said, they walked, rode or took public transport.
Finally in my experience, city kids are just as prone to underage drinking as their suburban counterparts, possibly moreso because of the increased opportunity. What they don't do however, is drink and drive.
I'd say this is one of the main reasons for the popularity of video games and child obesity. Kids have no way of getting around so they are stuck at home.
The main ingredient that you need for carfree kids to work is usable transit. And by usable, I mean it was frequent; if I missed my bus I would only ever be waiting 10-15 minutes for the next one. Everything I could want to go to would be reachable by bus in 40 minutes or less.
If you were to take a map of most American cities' bus networks and only showed routes that ran every 10-15 or better throughout the middle of the day on weekdays, an awful lot of them would be mostly blank. Even more if you were to require that for Saturdays as well, and pretty much all of them for the entire week. Provide the transit and people will ride; it's how Seattle managed to buck a national trend of declining bus ridership.
For me, a load of groceries fills 3 shopping carts in the store (packed to the top and overflowing) and 4 once bagged. That seems to be about 42 cubic feet (1.2 cubic meters) of food, with weight likely approaching a ton. I still have to buy a bit more during the week, since we go through about 2 gallons of milk per day and I'm just not going to attempt bringing home and storing 14 gallons of milk.
I use a 3-ton vehicle by curb weight. It can be a 5-ton vehicle when filled. The 5.4L V8 gets 11 MPG in the city. It's actually fuel-efficient on a per-person basis when I fill most of the 15 seats with people; think of it as 3 times a 5-seat vehicle that does 33 MPG.
Most people fit their week's worth of groceries in a grocery cart, and manage to push that around the store. I really don't understand why it's crazy to think you could tow them home with a bicycle. Plenty of people do this. It's a bit out of the norm in the U.S., yes. But don't for a second think it's impossible, or even particularly challenging.
