I don't have a car, and ever since Cole was old enough to wear a helmet, he and I have made a lot of trips by bike together. Riding provides a great opportunity to see and learn about the city, a practical way for busy parents to stay in shape, and an active and visible way to cut pollution and oil demand. Plus it's usually faster (and definitely more fun!) than taking the bus.
Unfortunately, I know a lot of parents, especially urban parents, are scared to take their little ones out on the road. If you fall into that category, I encourage you to take a look at the new Family Biking site The San Francisco Bike Coalition just put up, which has some really clear, practical information about equipment, safety, and good places to ride. It also includes a discussion forum where you can ask questions, like the ever-pressing "Will the Xtracycle fit in a MUNI bus rack?"
Hope to see you on the road!