One idea is to focus on balance first. Provide the child with a bike with no pedals, and a saddle height so they can put their feet flat on the ground. (Special 'balance bikes' can be had, but a normal bike can have its pedals removed easily enough). They should be able to scoot the bike along with their feet and learn to balance it, without being at much risk of falling over. Once they get the hang of the balance, add the pedals.