The boys have had bikes for a while, and, though I admit we've not used them much, they've been a fairly comprehensive waste of time. The big problem has been training wheels - they simply prevent the boys from learning how to balance.
So with the application of a bit of thought we've taken another route. First: lower the seats so the boys feel relatively safe. Second: remove the training wheels. Third: repeat the mantra "steer towards the direction you feel yourself falling". Finally: affix parent to back of seat to help out.
Over the course of a few days the boys have leapt forward. They can't ride yet but they are now very close - both have managed a few meters on their own. It will just take a little more practice.
I've now become a great fan of those wooden bikes without peddles you see around for little ones. I regret not having the kids learn to balance on those as a starting point.
(As an aside, the other problem with the kids bikes is that they are so heavy. The kids bikes are heavier than mine, which makes manipulating them a real pain for both child and parent.)