It's no secret that children know Santa's for real. Last year I found a site which sends personalised messages from Santa to your children. Mimi was 3 and a little too young to really get it, but I had another go this year and it worked a treat.
You upload a couple of photos, chose whether the child has been naughty or nice, add some personal details and then Santa says "hello Mimi, I hear you're 4 years old and you've been trying hard to be nice to your little sister...' and so on.
I organised it so that the messages 'arrived' for the girls while we were at Nana's, so they couldn't get suspicious about my involvement. The downside was that Nana's Internet connection was dreadfully slow so it took a long time to load the movie (so be warned) and as a result the girls got restless. It was however worth the wait. Mimi almost went into shock when Santa said her name and then opened his big book to reveal her photo in there. She held her breath while the elves pushed all the buttons on their machine and we all waited for the verdict of whether she was going to get the green-light for good... she did.
There are a variety of sites out there that can do this, I used the first of those listed below, a friend swears by the other. Perhaps a paper-letter would be nice, but in this digital age, the personalised video is a fun way to bring Santa-magic into the home, and especially nice while we're yet to reach the 'tricky question' stage.
Let me know if you have any Santa-message ideas.
http://www.portablenorthpole.tv/home
http://magicsanta.ca/homepage.html