I have sailed with Carnival several times. My wife and I really enjoyed our cruises with them. I hope you have a wonderful time. What cruise are you taking?
Carnival has a ship on a pedistal (trophy) that they give out to passengers when they win a contest or game when they play any one of dozens of games on board. The chip and pedistal are fully plastic and the word "Carnival" and the name of the ship is on the base. It is about 5" tall and 4" wide. If you contact Carnival, I'm sure they can mail you three of those trophies so you can wrap them up and give them to your kids for Christmas. Call your cruise counselor and ask how you can order them.
I won my trophy playing trivial pursuit on the "Carnival Spirit". There were about 20 of us playing and they gave out a first, second, and third.
There are enough nibbles and food on the ship that you won't need to bring any food on the ship. I think they are serving something someplace on the ship from 6:00 am to 3:00 am (ship time).
I hope you have a wonderful time. Good luck to you and yours.
ETA: McMama has an excellent idea with the puzzle. Carnival also sells puzzles on board, but i don't know if they will have your ship on a puzzle. I have seen ship jigsaw puzzles in stores. They probably just won't be your ship.