Measure their head. The circumference of the head 1" above your eyebrows.
note: If you dont have a soft tape measure use your cell phone charger code and then measure the cord.
If your head measures 20inches or under get a Mini Lifer Helmet
If your head measures 20.5" - 23.20" get an S1 Lifer Helmet
If your head measures 23.25" or more get an S1 Mega Lifer Helmet
And you will be use sizing liners to make your helmet fit tighter or looser.
The best thing to do is to order your helmet + a Fit All Kit for $8. You will get 7 Sizing liners of varying thickness to ensure a great fit. This is a great idea when sending a helmet as a gift.