Ahoy there, Teri! Welcome aboard planDisney! I'm so glad you're so close to your incredible Western Caribbean itinerary with a stop in
Grand Cayman!
I know searching the internet for information about travel documentation can be confusing, but luckily, we've got the word right from Captains Minnie and Mickey!
Regarding
Passport and Travel Documentation Requirements, for closed-loop cruises departing from a US port, US citizens currently need to provide either an original valid U.S. Passport, Passport Card, Trusted Traveler Card, OR an Original or copy of their state-issued birth certificate AND Government-issued photo ID. So the good news is that a passport is not a strict requirement as long as you can provide a birth certificate and photo ID! Grand Cayman is included in this requirement, and since nearly all Disney Cruise Line sailings calling on Grand Cayman are closed-loop, you're in luck!
That said, I HIGHLY suggest traveling with a passport whenever possible. If for any reason during your cruise it becomes necessary to return to the US from a foreign port, a passport makes that a much simpler process than spending hours, days - or even weeks - at an Embassy. It may be too close for your crewmate who doesn't have a passport to get one in time unless they were able to make an appointment with an in-person passport agency. If they're in Kansas, like you, the closest one is in Aurora, Colorado - which may be prohibitively far away, unless they happen to travel to a city with one for the holidays!
I have a friend that once was delayed for WEEKS in the Eastern Caribbean because he didn't have his passport and missed his ship after returning from an excursion not booked through his cruise line. He still tells the story about how he always thought being, "trapped on a tropical island would be a lot more fun than it actually was."
Well Teri, I hope that helps you chart a course for your Western Caribbean adventure! Thank you so much for your question, and if any others bubble to the surface while you continue planning, you know where to find us here at planDisney! We're always happy to give you a new heading!
Bon Voyage!