Little Things to Sew (which I have and love) but also a free download here.
The hats are reversible, but I let them choose their own fabrics from my stash, so the two sides don't coordinate quite as much as I would have liked, but the point was for the kids to actually wear them, so—success!
My kids have big heads. Louisa's is 21" and Leo's is 22". They are 6 and 4 years old! The pattern only goes up to a finished size of 21" (estimated for an 8 year old...), which is still too snug on Louisa's 21" head, so I had to do some enlarging. I also made Louisa's brim a little wider for style and coverage.
I free-handed an "extra large" around the pattern pieces trying to keep the scale the same as the difference between the medium and large. I basted one layer of Leo's together and it was still too tight, so I ended up using a 1/4" instead of 1/2" seam allowance and that made it just right.
I used a little grey's famous "no hand sewing" technique of sewing them together which was much faster than last time.