Tokyo is one of the most walking/biking friendly cities I've ever seen. Once you get out of central areas like Shinjuku/Shibuya, there are tons of lovely lanes where cars are technically allowed but so rarely go that the street is just filled with people walking. So you don't need to cart your kids everywhere. It's so safe that Japanese parents let their kids walk to the grocery store to run errands for them. Having grown up in suburban America, I hate the feeling of being stranded after school waiting for my parents to pick me up, or not being able to explore or go to the comic store in the shopping center myself. Once a kid is older than 5, Tokyo would be amazing.