To ensure the right helmet fit for a 5 or 6 year old you first need to get a measurement of the circumference of their head. The average head circumference for a 5 or 6 year boy is between 48cm and 53cm (18.8” – 20.8”). The average head circumference for a 5 or 6 year old girl is between 47cm and 52cm (18.5” and 20.5”). Most helmets that meet these size requirements come in size Extra Small which ranges between 52-54cm.