Hi Anne-Marie,
Six years old is such a great age to take a child to Walt Disney World. They are still young enough to believe in a lot of the magic, but they are old enough to walk on their own! It was not too long ago that my daughter was six years old, so let me offer you some suggestions based on my experiences with her.
Things that your daughter will like and things to stay away from depend a lot on your daughter's unique personality. For example, if she is really into the princesses and all things glittery, she is still at a good age to meet the princesses - such as Ariel at the Magic Kingdom or Belle and Aurora in France in Epcot. A girl who enjoys the princesses may also like to be made over as a princess at the Bibbidi Bobbidi Boutique.
Some girls are not in love with the princesses, or may have "outgrown" them by six years old. Is your daughter adventurous? She is probably getting tall enough to ride some of the more thrilling rides at Walt Disney World. You can start with The Barnstormer at Magic Kingdom. To ride the Barnstormer, a child only has to be 35 inches tall. If she likes that, you can start to work your way up to Big Thunder Mountain Railroad and Splash Mountain, for which she needs to be 40 inches tall. You can also gauge your daughter on how much she would enjoy a scarier ride. Many six-year-olds are ready to ride Haunted Mansion, and some would still find this attraction frightening.
I suggest that you order the free
Disney Parks Vacation Planning DVD. From that site, click on the link to order the Walt Disney World DVD. You can watch this with your daughter and see what she responds to in order to help you plan your time at Walt Disney World. Have a wonderful time planning your vacation Anne-Marie, and please do not hesitate to ask if you have any further questions.