Take this as you wish, but I'd seriously consider an alternative to stabilisers.
I hate them, for the fact that they fundamentally alter the dynamics of a bicycle to the point that they actually enable a child to learn nothing about how to balance on a bike. Add in drop curbs, uneven terrain and the inevitable training wheel breakage, and tumbles become so frequent you'll need shares in Elastoplast. And when you take the stabilisers off, they have to unlearn everything and learn to balance, to counter steer, to lean....
I had them on my very first bike when I was 3, and I never learned to ride without them. I didn't pick up another bike until I was 9 and had forgotten all about the evils of stabilisers.
The best bet, in my opinion, is to strip the bottom bracket out of the bike and let the kid get on with the business of learning to balance on two wheels without the distractions and interruptions from learning to pedal too, and without making the bike a quadricyle and totally eradicating the one aspect of bicycling they really "need" to learn to master the skills.
Balance bikes are the way forward, focussing the child on the aspects of balancing and counter steering. On a balance bike, a kid who has just barely mastered walking can pick up the skills required in less than a week, and have oodles of fun doing it. Then, later, you can add the additional complications of pedals, gears, brakes etc. which are then integrated in mere moments as the key skill of staying upright on two wheels is already embedded and second nature.http://youtu.be/nNDx5_1sZoo