Using the measurements given in The Knitting Workshop will give you a hat with a circumference of 18 inches at the ribbing increased to 21 inches.
I found this chart
with average hat sizes and it lists these sizes:
Child (3 - 10 years) Hat height =8.5"
Head Circumference: 19" - 20 1/2"
Pre-teens and Teens Hat height 9-10"
Head Circumference: 20 1/2" - 22"
It looks like the average sizes for a 10 YO would be a bit smaller than what you will get with EZ's numbers.
Can you measure the kids heads to see how close you would get with the 72 sts and the 4 sts to the inch gauge EZ gives in the book?
Then you can adjust your gauge by changing needle size or even yarn weight so you can get a better fit.
If you need to you can always adjust the number of stitches you cast on so you get the size you want.
EZ's patterns are very flexible and you can follow along with her patterns but fill in your own numbers to fit.
Hope this helps! I'm working on a couple of her patterns from another book but this hat is just what I was looking for to get some colorwork practice. Haven't done any in years.