You didn't say how old your child/ren is/are so if they are in diapers or trainers, make sure you bring enough as they are pretty expensive at
WDW and the dessitin and cornstarch for chaffed little bottoms. We also like the little mister fans. They are great for keeping the kids cool(and adults). Also a good supply of wet wipes. They keep you cool, clean, and anti-bacterial!
Have a great trip and remember you can always buy what you forget. It'll cost you more, but you don't have to go without!
Have a great trip!
SleepingB