Hi Krusheta:
What a great treat to visit Disney World during your cruise excursion. Even though your eight and a half year old has visited Magic Kingdom when she was four, I always recommend Magic Kingdom as the one Park you can never go wrong with if visiting just for a day. There is just so much to do at Magic Kingdom and your eight year old will likely experience the Park in a different manner now that she's older. While only a portion of the many Magic Kingdom attractions, our family always enjoys Dumbo the Flying Elephant, it's a small world, Jungle Cruise, Mickey's PhilharMagic, Peter Pan's Flight, The Many Adventures of Winnie the Pooh, Pirate's of the Caribbean, Tomorrowland Transit Authority People Mover, Haunted Mansion, and Monster's, Inc. Laugh Floor. Now that your 8 year old is taller and maybe even braver, she may also enjoy Splash Mountain, Big Thunder Mountain Railroad and Space Mountain. Your eight year old may even enjoy a princess make-over at
Bibbidi Bobbidi Boutique.
If it were me, I would travel to the Park via the cruise ship's excursion transportation method. This way, in the unlikely even that anything unexpected occurs during the commute, the ship would likely have a better idea about your whereabouts.
Enjoy your vacation.