My advice is to avoid training wheels, as they don't do much to improve balance. Instead, leave the pedals off of the bike and put the seat low enough to use feet to push the bike. Pushing and gliding will develop balance skills, while still having the feet in position to keep from falling over. It's the way both my younger sisters learned, and they picked up riding quickly. Once the young'un has mastered balancing on the bike, then add the pedals.