I adjusted the pattern to fit my 6 year olds hands.

I used a size 1 (2.5 mm) for the ribbing and then switched to a size 2 for the main part of the mitt.
I also started following the chart on row 9 and then started the thumb 10 rows later.
To finish the thumb, just picked up the stitches with the smaller needles and did the ribbing.

My son is super excited to wear them this fall. What a great pattern for a boy.