Its great when kids are young enough, and not jaded yet, to be innocent enough to still believe. The first Xmas with my wife and our eldest I was closetted in the attic for a couple of weeks making him a wooden train set (Thomas the Tank engine) and Xmas morning he came down to find it in the living room with complete with footprints, and empty milk glass and cookie crumbs. The wide eyed joy on his face is something I'll treasure for eternity...
Its moments like that that make all the grief your kids give you over the years fade into insignificance.