I think you should give them out editable pretty URLs made from title of the post such as pen.io/this-is-a-test-page instead of sub-domains. This will also help you with the SEO.

Assume that people will want to build lot of pages. Don't let the lack of sub-domains hinder this. Plus, in future you can let them build a blog or something from this set of pages. Advantage you have is you are letting them start-off with minimum resistance.