Bicycle Helmet Safety Institute: Teaching your Child to Ride a Bicycle: A BHSI article with a focus on safety.
Bicycle Science - Losethetrainingwheels.org: Describes phases of learning to ride a bike, issues of stability, and references for further reading.
Bicycling for Beginners: Learning to Ride: Advice for beginners on starting to ride a bike.
BikeSense: The British Columbia Bicycle Operator's Manual, this globally applicable manual gives practical tips on how to ride your bike in urban environments.
Teaching Kids to Ride: Article by Sheldon Brown that discusses the finer points of teaching children how to ride a bicycle.