The child's head circumference determines the necessary helmet size. Many models are adjustable, for example by means of a small adjustment dial at the back. This is of course particularly useful for a children's riding helmet, as children are still growing. It's important for the child to try on the helmet before wearing it on a ride. The helmet should not wobble or fall off the child's head when he or she shakes it, even when the chin strap is open. On the other hand, the helmet must not be so tight as to be uncomfortable. If your child finds the helmet uncomfortable then he or she would avoid wearing it whenever possible. You should therefore make sure there is adequare padding provided by the helmet, so that it's comfortable to wear.