How about turning it into a regular web service which parents can use to upload any photo and turn it into a coloring page (PDF) they can print out. You could

How about turning it into a regular web service which parents can use to upload any photo and turn it into a coloring page (PDF) they can print out. You could have a "free" service for producing, say, one paper a day, and charge for version with more. Any parent whose child likes it will want to buy it. You could copy some of patio11's ingenius marketing tactic - have a set of pages (done using your tool from freely available (or purchased) photos) that people can download, and hook them on the app.