I'd strongly suggest getting one of the free or cheap training wheel bikes and turning it into a balance bike yourself by removing the pedals. One of the benefits of this is that you actually have a bike that can be used either way. I set out on this path for green reasons ($ and enviro), but it turns out one of my twins only wanted to ride with training wheels and the other only wanted a
I promise that is was VERY easy. I followed the directions here. The only real challenge is if you want to get the chain off you need to find someone with a tool to pop it off. That might mean your local friendly bike store, or in our case a friend who is into cycling (thank Joe!).
Oh and I found this site is a great resource for thinking about teaching your child the bike skills.