Teaching kids to cycle can be sometimes a daunting task. If a child gets hurt while trying to learn how to cycle, this can discourage him / her from it for a long time. I have known a guy who was afraid of cycling because of his childhood accident and he only fully got over this by the age of 16!!!
First step should of course be a three-wheeler. It will teach him / her the motions and the balance. Taking your child to a cycling trip with one of both parents could be an amazing experience too! Make sure he / she wont overstrain himself / herself trying to catch up with you!