Ansam, welcome back again! I love to help, so please, keep the questions coming!
If there is one thing that Disney Cruise Line does well, it's providing more than their fare share of shopping opportunities onboard! You will find a variety of princess dresses in many sizes, certainly one just right for your daughter, in Mickey's Mainsail shop. The shops are only open while the ship is at sea, due to regulations in the different ports, so keep that in mind when you plan to purchase a dress.
Another idea is to check out the
Disney Store's website before you set sail. Quite often, you'll find that they are offering free shipping and a variety of sales, so you might even pick out the perfect princess dress ahead of time for much less cost. Prices change frequently, but as I write this, they are offering 25% off costumes plus free shipping. Believe me, those things can be rolled up tightly and packed easily. I know from much experience packing many princess dresses for our trips! You could hide it in your luggage and bring it out once you're onboard the ship as a surprise. She won't know that you didn't buy it on the ship, and you'll be happy to have saved a few bucks! Sign up for their email list to be notified of future promotions.
I'm always happy to help with other questions, Ansam! Keep me updated on your plans!