One word of advice to families planning a trip to Disney World: it is better for all involved when the child to whom the trip was primarily geared is not feverishly, barfingly sick for the entire trip. (If "barfingly" isn't a word, it should be.) It also helps when every other members of the family do not alternate being sick immediately before, during and after the trip.
For more controllable advice, the House of Manhattan officially recommends this book for use in planning a Disney vacation with kids.