From the website: By changing weight of yarn and hook size, these instructions will make helmets to fit a range of head sizes:

Newborn to 3 months -14 to 15 1/2” head-
Sport weight (24 stitch) yarn with 3.50 mm hook.

3 months to 1 year -16 to 19” head-
DK weight (22 stitch) yarn with 4.00 mm hook.

6 months to 3 years -18 to 21” head-
Knitting Worsted (20 stitch) yarn with 5.00 mm hook.

Use worsted weight yarn -
5 mm hook gives gauge for larger size (7 stitches to 2 inches)
4.5/4.75 mm hook will give gauge for the smaller size (4 stitches to 1 inch)