The problem of creating vibrancy can be redefined, at least in part, as the challenge of designing for children.
Here's a few initial design goals this approach naturally leads to:
1. streets that you can play on
2. walkable neighborhoods (kids can't drive) - specifically parks within walking distance
3. education as a central experience (Christopher Alexander writes about how to utilize patterns around education to create vibrancy in " A Pattern Language" as well as how to make other city experiences work for children)
4. the city as an education tool
5. safe, friendly neighborhoods (i.e. Jane Jacobs' "Eyes on the Street")
By asking the kids, I think more design objectives could be found and concepts could be tested. In fact, it may be helpful to attempt to actively bring input from children into our evaluation phase of this challenge.