I like the idea of having it verbal, and I agree that it would be a quick and simple way to test the idea! However, I do think that there needs to be some form of illustration accompanied with it. Depending on age, some children don't like listening to even the most interesting story unless there's visual stimulation. It doesn't have to be complicated, but I think the test response would be stronger if there were visual aids. What do you think?
I see where you're coming from! If you want a fast prototype, then I think self-printing would be the best option in making a physical product. Printing the story on stationary, and binding by hand should be a quick and concise thing. There are also a few places in the city that specialize in binding. In my opinion, creating the prototype should not be overly difficult or time consuming, unless illustrations are involved; that could add some time. Have you thought about potential ideas for the story? If you like, I can do some research and brainstorming, and throw a couple of ideas your way. Also, I think schools or daycare centers are a great place to start to test this out.