How tall is your child? Although age is the general guideline for selecting a bike, height is the most important factor to consider when choosing the correct size bike for your child. An easy way to check for fit: The child should be able to stand over the bicycle with about an inch or two between the cross bar and his or her crotch. About.com's guide to bicycling posts a handy chart for choosing a properly sized kids' bike.
What's your budget? Just like full-sized bikes, kids models come in a wide variety at a wide range of prices. Also like full-sized bikes, quality often correlates directly with price.