I am working on a hat for a 4-year old right now. I've spent my life around kids - baby sitting, volunteering...but I've never had to knit a hat that wasn't part of a pattern or for a generic adult head.
How big are 4-year old's heads?
I am the youngest in my family...there are no 4-year olds heads to measure...
Google helped me, of course. I googled the phrase "circumference of a 4 year old's head" and Google managed to find this great website: http://kidshealth.org/parent/growth/growth/growth_charts.html
Who knew growth charts for kids height and weight might also have head circumference. Low and behold, they did!
Very helpful. The charts with head circumference only go up to 3 years of age, but that's close enough for me!