One other problem with mass-transit and children. You have no control over the environment they are going to be exposed to. For all you know you are going to end up on a bus or train with people swearing, talking about who knows what, or be subjected to some less than appropriate advertising.
Unless you're home-schooling your kids, you don't have that much control over their environment. If it's not some random person on a bus, then it'll be their friends at school. Either way, they're going to be exposed to less than appropriate material.