Welcome aboard, Michael! May the Force be with you! You must have a Star Wars fan in your family if you're asking about traveling with a lightsaber. I can tell you from lots of experience that there will be lightsabers and plastic toy pirate swords aplenty onboard for both the Halloween events as well as for Pirate Night.
Two of Disney's most popular costumes are pirates and Star Wars characters, so you may just see a few Chewbaccas lounging by the pools or mini Captain Jack Sparrows darting through the halls on
Pirate Night! I suggest, however, that any lightsabers or toy pirate swords you plan to bring be packed in your checked luggage, both for your flights and for your boarding onto the Dream.
As long as your child, (or maybe this is for you!) is responsible with his or her (or your!) lightsaber while onboard, it'll be a really fun Halloween! Don't forget your camera because even the Disney characters get into the fun and will be out and about in their own Halloween costumes!
Have a wonderful time when you set sail next month for what is sure to be a spooky and fun
Halloween on the High Seas cruise, Michael!