Two weeks before my son's fourth birthday, I took off his training wheels and pushed him down a gentle but long grassy hill. This was so he could just coast downhill and think about balancing and not worrying about pedaling at the same time. He looked like an old pro after about fifteen minutes.
The hard part for kids doesn't seem to be balancing as much as learning to stop the bike without falling down. Grass helps with that, as it's softer than falling on pavement.
Once they figure out how to put a foot down when stopping to prevent tipping over and then how to get started again without a push or a hill to roll down, they'll have it for life.