On the main pathfinder society page, click the link that says "Join the Pathfinder Society and create your character now!" from there, click GM/Event Coordinator and then under the heading Pathfinder Society cards, you can reserve 10 PFS #s, you do not need to have a pathfinder society account to play PFS, just a #, so even though their name is not on the cards, they do not have to be registered until they are old enough to have their own accounts.
I'm not certain this solution will work. I could have sworn that I read that when you reserve PFS numbers, those numbers eventually get recycled back into the system if unused. If that's true, it may produce unexpected results.
I could be off my rocker, in which case I'm sure someone will set me straight.