give them a 1 and an 8, outlined, as large as you can print them out, give it to them, have them sign inside (make sure they don't sign over the outlines) and scan them, save as bitmap tiffs so you can place them and color them if need be. I wouldn't bother converting to vectors, just be sure to scan at a high resolution, minimum 300 ppi. But Pierre's answer for setting text in a custom shape will work, but, there is a ton of editing...
Steve