If you are looking for a princess dress to wear in the parks, WDW has a rule that costumes are prohibited (except for the halloween parties) for ages 10 and up. I include the language below. Not sure how strict they are with this rule.
Disney World Guest Appearance Guidelines
Disney World reserves the right to exclude visitors to their gated attractions who wear clothing that could create a distraction. This policy is in effect at the Magic Kingdom, Epcot, Disney MGM Studios, Disney's Animal Kingdom Theme Park, Blizzard Beach, Typhoon Lagoon, Pleasure Island and Disney Quest.
Examples of unacceptable attire includes:
Clothing displaying offensive messages/language
Adult costumes or clothing that can be viewed as a costume (children under age 10 are excluded)
Clothing made with offensive material, (ie transparent)
Clothing that is excessively torn
Clothing which exposes inappropriate portions of the body such as string bikini tops, G-strings, bikini bottoms, etc.
Guests wearing wedding attire are discouraged from entering the Theme Parks.