Here is the one I wrote for DD:
Dear DC,
Are you getting excited that Christmas is on it's way? Mrs. Claus and I sure are! We are very busy getting ready for the big night. I'm sure you know about the elves that work with me in my workshop but did you know that not all elves make toys? Now that you are old enough, I can share one of my secrets with you.
Every year I send some of the younger elves on special assignments. These elves visit the homes of little boys and girls the world over, spend time with their families, watch over the children and learn about human family life. This year I have sent one of these special elves to you! In order for her to gain the magic that she needs to do her job, you will first need to give her a special elf name. Once you have named her, she will be your special Christmas elf forever.
While she is visiting, her job is to watch over you and your family everyday and return to the North Pole each night once you go to sleep. She will report to me on how things are going and what she has learned and then be back at your house before morning, waiting for you to wake up.
In order to keep the important secret from being accidentally revealed, I have used special Christmas magic to make your elf look just like a toy during the day. You are welcome to pick her up and hug her but please be careful. She is very delicate while in her toy state. At night you will need to leave her by a window so it is easy for her to get to the North Pole once she comes alive.
I have to warn you, however, that these young elves can be quite mischievous. If you catch her making any mischief, please make note of it. Also, because she is such a young elf, she will return to the North Pole with me after Christmas for further training I will pick her up when I visit you on Christmas Eve but don't worry. I will send her back at Christmas time next year.
I hope you enjoy having your own special elf this year. She will be watching you everyday, so make sure you continue being the good girl I know you are.
Love
Santa