If you're outside SF itself, look for the old parts of a town, with houses built in the 20s or earlier. There's probably a "main street"/downtown area near by, and because it's an old part of town, the streets are narrower and traffic moves slower.
Much of suburban Marin County is a paradise in this regard - the Ross Valley cities of Ross, Larkspur (not larkspur landing), Kentfield, San Anselmo and Fairfax[1] are full of kids of all ages riding their bikes and walking to school every day.
Berkeley and Albany. My neighbors told me they can't get their teenager to finish his drivers ed course because he just doesn't care that much about driving. His school, the park and his friends are all walking and biking distance away.
Thanks for the link. This goes in the right direction, but so far I have not been able to identify truly walkable neighborhoods through websites. The main problem is that the metrics usually being used, do not reflect the type of walkability that would be beneficial for raising children.
As an example, I randomly picked a particular address on walkscore.com (https://walk.sc/37yZciv) which gets a walk score of 91 (walker's paradise), but that particular address is on a large 7-lane street with not a lot of walking infrastructure around. I'm guessing the score might mostly be driven by proximity to stores, which doesn't necessarily matter in this case. Better metrics may be such things as amount of car traffic, noise levels, size of sidewalks (in comparison with size of streets), amount of small streets and distance to larger roads, etc..
I wonder if a website exists that puts more emphasis on how nice it is to walk in a neighborhood vs. mere practicality?
Milpitas really is not walkable, although you can somewhat decently navigate it with a bike. Walkability mostly is a function of density, which you can just glean by eye from a satellite picture. Unfortunately, many otherwise pedestrian-friendly places in the Bay Area have homeless camps set up on them, which might be a deterrent.
Anything south of Oakland/SF is mostly strip mall hell, but parts of Alameda County, Contra Costa County, Marin County and San Francisco might be what you're looking for.
Noe Valley/Bernal Heights is quite good. There is also schools in Noe but I guess SF is some kind of lottery system so it's not guaranteed your kids can go that school.