The following is from Sheldon Brown's website:
http://www.sheldonbrown.com/teachride.html
The Undersized Bike Approach:
The ideal bike for learning to ride, whether for a child or a deprived
adult, is a bike that is "too small" for efficient riding. For learning
purposes, the rider should be able to sit on the saddle with both feet
flat on the ground and the knees slightly bent. The bike can then be
used as a hobby horse or scooter, with the feet always ready to stop a
fall. It may even be useful to remove the pedals at first, so that the
feet can swing freely. Ideally, a bike for this approach should have at
least one hand brake, so that the child can stop while using both feet
for balance. A good place to practice is on a grassy field, perhaps
with a slight downgrade.
Unfortunately, it is often difficult for parents to justify the expense
of a smaller bike that will be outgrown shortly, so there is a constant
temptation to buy a bike that is a bit too large on the theory that the
child will "grow into" it.
Scooters:
A scooter can provide an excellent alternative way to learn balance,
and is probably a better alternative to training wheels, especially for
a child who has been depending on training wheels too long. A scooter
separates the steering/balancing function from the pedaling function,
so the child can concentrate on learning to balance without the
distraction of pedals.
An additional advantage of a scooter is that it is less scary than a
bicycle. One foot is on the ground much of the time, while the other is
only a few inches above the ground with no obstruction to a "bail-out".
Any child who can steer a tricycle can operate a scooter at some level
of proficiency. To start with, one foot can maintain nearly constant
contact with terra firma. As skill is acquired, the pushing foot
gradually spends less and less time on the ground, until the basic
skill of balancing on two wheels has been mastered.
