Helmet sizes are usually measured in centimetres around the circumference of their heads. That's true for adult helmets as well as kids' bike helmets.
The best way to measure your child's head size is to use either a cloth tape or a piece of string. If you use the string method, just use a ruler to measure the string's length. Measure around the largest section of their head, horizontally just above the ears, making sure that you take in the brow and the occipital lobe, the bump on the rear of the skull.