Hi Janet, How special to be celebrating a Birthday at Disney World!
When you arrive at a Park, be sure to stop by Guest Relations. You'll be able to request a complimentary, "Happy Birthday" pin. It makes a fun souvenir, and Cast Members will go out of their way to wish your daughter a Happy Birthday.
I would suggest that you make dining reservations for a place such as Chef Mickey's. See
HERE. They have a wonderful atmosphere and it's a great place to celebrate a birthday. You can order a personalized cake in advance by calling: 407-827-2253. Chef Mickey's has a fantastic "make your own ice-cream sundae" station, which my children love. If your daughter is a Princess fan, you may wish to celebrate at Cinderella's Royal Table instead. Call 407-WDW-DINE (407-939-3463) to make your reservations. You can call up to 180 days in advance.
Hope this helps and Happy Birthday to your little girl!
Kaylene