Diana, celebrating a birthday onboard one of Disney's pixie-dusted ships is the best way to celebrate, in my opinion! I had my birthday onboard the Wonder last year and it was one of the best ones yet!
As you know, Disney Cruise Line offers a few different
birthday celebration packages, available for pre-order online. On that same page, you'll find the order form for specialty cakes, which can be presented either at dinner or in your stateroom.
If you'd like to bring your own decorations, I suggest magnets. On the ship, you'll see many guests decorate their stateroom doors with personalized magnets they've purchased or printed on magnetic sheets at home. It's not only a fun way to celebrate, but also a great way to help recognize your door in the long halls of identical stateroom doors.
You are certainly welcome to bring your own party hats and decorations, however do be aware that Disney Cruise Line has a
decoration policy stating that guests may not use adhesives, sticky or tape, to adhere decorations. They also ask that no over-the-door hangers or organizers be used as those all tend to damage their doors and walls. Stand-up decorations for tabletops would be perfect to bring, as those usually fold flat for packing. Or, bring some balloons to blow up and spread around the room one day! If you'd like to hang a banner, you could do so with magnets. Another option is to drape a banner over the bed, so your little one will see it as soon as he enters the stateroom. You could even tuck ends of a banner into cabinet doors on each side of the mirror, so removal would be as easy as opening the cabinets - no adhesive necessary.
I'm happy to help with other suggestions and ideas as you continue to plan, Diana!