Ahoy, Rodney!
Thanks for stopping by the Moms Panel with your question.
I'm happy to hear you and your wife are planning to take advantage of the "semi-formal" night to dress up and take photos around the ship. Since you say your wife's dress is "Cinderella-like", and not an actual costume, it will probably be fine to wear that evening. You'll see guests in everything from sundresses to ball gowns on semi-formal, formal, and dress-up nights. If it's more of a "costume", you might feel a bit out of place. That being said - this is a Disney cruise, and guests will probably appreciate your commitment to the Cinderella theme!
If you choose not to wear your outfits to dinner that evening, you can always bring them along to dress up for photos around the ship. The ship's photographers have lovely backdrops set up each evening in the lobby, and there are several photo packages available for purchase onboard.
If you enjoy dressing up as your favorite Disney characters, you might want to consider visiting the Future Cruise desk while onboard and booking a
Halloween on the High Seas cruise! You'll not only get a discounted rate on your cruise if you rebook onboard, but you'll be able to enjoy a sailing with lots of opportunities for costumes and dressing up!
I hope this helps with your packing and planning! I hope you have a most magical time on your Disney Cruise Line vacation. - Diane W.