I love this idea... and maybe I'm considering stocking up for my future children: The Official Tooth Fairy Kit from Notion Farm.
Even their description & instructions are awesome. The Official Tooth Fairy Kit contains one letterpress Certificate of Record for filing with your local Tooth Fairy and one reusable silkscreened cloth deposit bag for your first and all subsequent transactions. The official certificate keeps record of name, age, tooth lost, method of extraction and compensation information.
You can buy it here for only $16.