Our little one isn't here yet, but I'm three years older than my brother and after "all was revealed" to me, I spent the next 2-3 Christmases helping keep it alive for him... by answering his letters to Santa, complete with reindeer hoofmarks for "authentication".
My brother and I still swear we heard bells on the roof one Christmas morning, but our parents have never fessed up to that one! hehe