How about a picture of the child with a missing tooth, on printable fabric? If you can get him or her to hold the tooth up and point to the empty spot in his/her grin, that would be perfect. If you're into embroidery, you could add a little saying, which I've seen on some of the machine embroidery websites (but could be done by hand, too). Google will probably be the fastest route to one of those. Sorry I don't have a specific example.