Hat size

Hey everyone! I want to make a hat for my 10 year old little cousin, but I’m not really able to find a pattern that’s specifically for this age. I’m pretty sure his head is average sized (lol?) and I just want it to be simple with a little bit of Fair Isle colorwork. I have no problem coming up with my own type of design, but I’m not really sure how to get the sizing right.
Any pattern help?

“small” adult size should be okay - especially if has a ribbed and/or otherwise stretchy brim/edge