Here's what I would do...

Take off the the training wheels, pedals and cranks off the bike. Lower the seat until they can push the bike along with their feet. As they get better at scooting along with their feet, slowly raise the seat. Let them practise a bit at the new height for a bit. Then raise again. Repeat this until they are just on their tip - toes as they scoot along. Once they get to this point, put the pedals and cranks back on the bike & watch them go!

This is the fastest and safest way to teach anyone how to balance a bike. (In my humble opinion).