my favorite hat sizing chart here as well. Tot Toppers, where the chart originates, features "original patterns for tots & a modern line for those who are not". The patterns spread much farther than just hat patterns, but I've found many of my favorite hat patterns for kids on their site!
My most used parts of this chart are the first few lines, which feature head circumference and length. I will simply subtract 2" from the head circumference to know which size hat to make when using someone else's pattern, and aim for these sizes when writing my own.