Welcome aboard, Madeline! I'm glad you've found the Disney Cruise Line section of the Disney Parks Moms Panel to be helpful as you plan your exciting sailing on the Fantasy!
The dress code for dining that particular day is sometimes listed as casual, but I guess what it should really say is "Pirate Casual", as pirate attire is highly encouraged all evening long, including in the main dining restaurants. Now, if you have Palo or Remy reservations that night, you wouldn't want to wear your buccaneer gear there, as those adult exclusive restaurants have a strict dress code. But, in the other restaurants, you should absolutely wear your pirate gear!
In fact, even your dining servers will have on their pirate-y best that night. Either prior to dinner in your stateroom, or at the dinner table itself, you'll each receive a Mickey's Pirates IN the Caribbean party bandana to wear. Be sure to stop by the atrium before or after dinner for some great photo opportunities with the Disney characters who go all out in pirate costumes that night too!
Then, after a pirate-themed menu at dinner, it'll be time to head up to Decks 11 and 12 for the
Pirates IN the Caribbean deck party and fireworks. It's such a fun night, Madeline! Don't forget to pack outfits for the adults too - the costumes are not just for kids!
Please let me know if I can help with anything else before you set sail on the Fantasy!