Talk to your customers. Ask the 10% who are repeat visitors why they came back. Ask the 90% who don't why they didn't come back. It could be that people visit for the novelty but don't find value in a recurring subscription. Maybe you have the wrong target market (maybe schools, daycare centers, or child-oriented businesses like Gymboree or Chuck E Cheese would be better targets). Also, printers suck...they waste paper and ink is expensive. I would try the mobile app, but let me color directly on the app (I snap a picture on the iPad, give it to my daughter, and let her fingerpaint on the screen, minus the messy cleanup). Then offer a service to have the result professionally printed on glossy paper, framed and shipped to me. There's a possible revenue stream.