Royal Greetings Tammy! Sounds like your daughter is in for the birthday of a lifetime and kudos to you for planning something special in advance.
Actually both scenarios would work out fabulously. You can certainly contact the Dream Makers over at Disney Floral and gifts to see what kind of magic they can pull out of their sleeves. They specialize in
"In-Room Celebrations" that are sure to make any girl feel like a true princess and they can really something special for your daughter whether it be simple or over the top.
You can also contact the Disney Cake Hotline to discuss having a custom cake delivered to your room, but this depends on where you're staying. Cake delivery is only available to resorts that offer in-room dining, so you would need to contact (407)827-2253 to discuss availability at your Disney Resort. They will also be able to assist in ensuring balloons accompany the special cake.
But I must say, I honestly think the "Royal Highness" package at Cinderella's Royal Table is such a great idea. This package includes a 6" princess cake (choose from Aurora, Cinderella, Ariel, Belle, or Tiana), table decor, a mylar princess balloon, and a beautiful tiara. Imagine your daughter's face when you arrive at your table to find that the princesses have made special arrangements just for her birthday! The table will be sprinkled with pixie dust and she will be presented the sparkly tiara on a velvet pillow by a Royal subject. To book this package please call (407)824-4477.
As far as leftover cake is concerned, if you have any, your server will happily box it up and place it in a bag for you upon departure. Your resort room will have a mini-fridge so you'll be able to store it when you return!
Best Wishes,
Amanda F.