Balance bikes are the trend in teaching a kid to ride a 2 wheeler. The idea is to allow them to learn balance before they have to coordinate pedaling. There are very nice custom bikes built specifically with this in mind. However, they tend to be a little spendy and when your kid is ready for pedals you've got no choice but to buy an entirely new bike.
There's a pretty good background of what I'm talking about here:
Step 1: Tools and getting started
To do this you're going to need a 12" adjustable wrench, a chainbreaker, and a screwdriver. If you have a used bike you're going to want a pedal wrench.
Our objective is to remove the chain and cranks. If you're starting with an assembled bike, remove the training wheels and the pedals - note that the left pedal is reverse thread. You should probably use some penetrating oil before trying to remove them.
Step 2: Remove the chain guard and chain
Once you have access, use the chainbreaker to remove the drive chain.
Step 3: Remove the crank
Next turn the bike around and take a look at the way the cranks are held in - on this bike it is a one piece crank held in with a single nut that was reverse threaded. Remove the nut, washer, and the bearing cup. Remove the bearing (this one is caged, thankfully) then remove the crank.