I thought a balance would be great, but neither of my kids liked the balance bike we bought. They rode with training wheels for quite a while.

What got them riding without training wheels was learning to ride Razor scooters. One afternoon when I saw them both riding sweeping S curves on their scooters, I took their training wheels off. At 6 and 7 1/2 yrs, they when from total terror on their bikes to riding comfortably in a few hours. It only took a few rides with me running alongside holding on (continually holding on less each time).

It was amazing.

But riding with training wheels looks miserable... it destroys the smoothness and joy of a bike. I'm surprised kids want to persist with it at all... My kids loved balance bikes (i.e. normal bikes with no pedals) because they don't have the resistance and weird balance issues of a tiny third wheel.

You can start balance bikes at a younger age.

There are great videos of a Swedish three year old on a half pipe on a balance bike! My kids were not so fearless, but my son did a 8 mile ride (flat ex railway line) on a balance bike when he was 3. And before anyone accuses me of pushy parenting... we didn't make him! The plan had always been for my wife to ride with him separately, but he insisted on coming the whole way with his two siblings. It was a really memorable day.