After much banging my head against the wall (not literally, but figuratively) I have implemented a sizing chart for hats, scarves and turtles.
This should save you some pain staking work trying to secretly measure a little person's head and then somehow change the subject as they incessantly inquire as to why you are performing such an activity involving the circumference of their noggin.
It will also help me to help you find a size more suited to the age of the person whom will be receiving the item.
Thus, with no further ado, I have a page entirely dedicated to such information - Sizing Rubric.
This information is also posting on my shop - so no worries, you don't have to go to multiple sites to get the information you need - it's all there for you!
In the meantime, enjoy your December.