I am working on a new version of the pattern with 3 larger sizes (child, adult small, adult large), and it should be ready later in October.
In the meantime, here are some tips to enlarge the hat for an older child (remember that you’ll need more yarn for larger hats):
If you think the largest size might fit around, but want it to last longer, make the ribbed brim about 3 inches long. You can fold it up at first, then let it down as you need more length.
For a toddler size (18” around) hat, cast on 84 sts. Add 4 rows to the brim, and 2 rows to the seed stitch section before the cables. For the rest of the hat, work as for the largest size with an extra repeat (that is, 7). You may want to try the hat on the child before beginning the decreases, in case you need a little more depth.
For older children, it becomes a little more difficult to modify the pattern. If your child has a small head, you can try Tip 2, adding more rows to the brim and seed-stitch sections. If your child has a larger head, add another pattern repeat (so cast on 96 sts, work 8 repeats) and work a few more rows in the brim and seed stitch sections.