Most training wheels are so poorly designed that they make learning to ride harder, not easier.
Find a grassy hill with a gentle, long slope. Put the seat low enough that the rider can easily put a foot on the ground. Put the one pedal at the "noon" position, out of the way. Then let the child just "scoot" down the hill, using feet instead of pedals to push the bike. In an hour or two, most kids will be ready to coast down the hill without using their feet. After getting a good sense of balance, using the pedals is an easy next step.