As I recall we measured the wrist and bought the one closest to his size.
On the site I provided they have listed wrist circumference sizes:
Small up to 6''
Medium up to 7''
Medium/Large up to 7.5''
Large up to 8''
I would measure and see if the small one would work, it has been 9 years since I had a 9 year old wrist in the house.