Hi, Amy!
You are about to arrive in the most magical place on Earth...truly! Your older girls will love their trip to Walt Disney World, while you and the baby will also find so much that is geared just for your needs. Of course, Disney thinks of everyone in the family.
My absolute favorite source of information for anyone who is bringing the youngest Guests to Walt Disney World is the page entitled
Little Ones at Disney Theme Parks. There, you'll find a list of attractions that even the baby can ride, as well as information about Baby Care Centers, available in each of the four main parks (Magic Kingdom, Epcot, Disney's Hollywood Studios and Disney's Animal Kingdom.) Each of these Baby Care Centers includes a lovely room specifically for nursing moms and their babes. Low-level lighting, comforting rocking chairs and a nice, quiet environment will provide you with the perfect spot for feeding the little one, with no distractions.
Try to time the older girls' exploration of a part of the park that's not as baby-friendly with your nursing schedule. This way, two of you will be quietly enjoying the Baby Care Center while the others are enjoying a thrill ride or a parade or show that they are sure to love. Meet back up afterward and continue your family tour of the park. You'll love it.
Send more questions as you think of them, Amy; we love to help!