Playschool post?

I walked into my sons playschool today and noticed they have put a postbox in the corridor for the kids to post christmas cards to eachother. He is only 3 and a half years old? Surely thats a bit young to have to post christmas cards to other kids? I actually wasn't planning on getting any cards for the other children at playschool, when a friend told me she had written 70 cards for her kids playschool mates I told her she was crazy. Now i'm confused, am I meant to buy cards for all the kids in his playschool? Or just the ones he plays with? To be honest he tells me he plays with children that don't even exist so getting him to name the kids he wants to give a christmas card too would be rather difficult. Its a catch 22 situation, I either don't buy cards and feel terrible guilty when he gets cards from others or I spend money on tons for him and he doesn't get any back. They give out all the cards at the end of term so its not like I can wait and see who gets him 1 first.