Take some time on the Internet and research coop construction. Also, there are so many free plans out there; go to Purina company's website for a free coop plan that you can even download. Check out free books at your local library and/or purchase Building Chicken Coops for Dummies.
Basically, the chickens need a nesting box(s), a coop itself (2 square feet minimum per hen), roosting bars, (looks like a ladder) and a run. It may seem overwhelming at first, but simply pick and choose ideas and adapt them to the existing playhouse/fort structure. Look at coop manufacturer sites for additional ideas, photos, layouts, etc.
Good luck with the project-it will be a memorable event and the conclusion will be something you may all enjoy as a family.