Children's bikes are generally measured by wheel size, rather than by frame size. Remember children do grow very quickly and you need to take this into consideration when buying a kids bike, However, if you buy a bike that is simply too big for your child, you will compromise your child's safety, and ultimately discourage your child from cycling.
Age of Child
Which Size Bike Do I Require?
Adults bikes and those for 10/11 years and over, are generally measured by frame size. This is normally the measurement between the centre of the bottom bracket to the top of the seat tube.