I know that there have been digitizers in the past who made designs for tooth fairy bags. A Design in Stitches and Dollar stitch have some designs for tooth fairy bags. I think that making a small bag out of fabric in the colors they like with a design of something they like would fit the bill. You can even put For the Tooth Fairy on the bags to be sure the Tooth Fairy knows the coins go in the bag. Have fun coming up with the bag and design.

check out the tooth faries at
http://www.kreativekiwiembroidery.co.nz/

If you are on facebook, join Embroidery Garden and like page, Reen has a monster tooth fairy pillow available for free download. It is pretty neat.

Years ago, I embroidered a tooth on the top of a piece of felt. Then took a 3 sided pocket design and stitched that around the tooth piece and a bottom. Used pinking shears to cut it out and had my tooth bag. Best part of this is that you can make it any size you want. I gave them to kids who were loosing teeth. Parents liked them. Fun to make.

There are some really cute fairies to stitch here. Maybe one of them would work.

2 posts with links to tooth ideas and designs. They are old so I can't promise that all the links will work.
I sent you a pm.

Also if you belong to sew forum and search their site by topic, you will find some teeth and also some sayings that may work for you. I love to search for designs there, you just never know what to expect, it's a very easy search.