Being the mother of two very large headed children, I always buy a size up, so I got the 2-4 year size. This hat is extremely big on my 21 month old.(His head is in the 98th percentile). I've had this experience with another hat from this company as well.

It's cute, he likes it, and for now I am just pinning it shut a little smaller. I guess that means we'll get more years out of it. However, if you have a normal headed child, you might want to consider getting a size smaller, or know that this really is for 2-4 year old. My big headed 4 year old can wear it just fine!