From my understanding, you cannot have a professional photography session done in any of the Disney World parks.
You might be able to do something in the parks if you use Disney Fine Art Photography & Video....However, I've heard that the in-park sessions are reserved for wedding couples only. The only other choices are around the resorts.
Most photographers in Florida will
not agree to photographing in the parks, as they could be sued by Disney, banned by Disney from their parks, and lose their credibility as photographers overall.
The photographer I used for my engagement photos said no to the parks, and so we chose the Boardwalk area instead. They turned out wonderful!
However, you will get some photographers that are willing to be secretive and go into the parks to get you the photos you want. They will charge you a session fee
and you will have to pay for their park ticket(s) as well.
It's up to you on how you wish to go about it, but let me just say that you can still get some really wonderful photos around the resort areas....The Boardwalk and Poly are some of the popular spots for photo sessions and because it's a resort you'll get a more relaxed setting allowing you to get the most of your photo session.
Now if you're talking about
Disneyland, well all bets are off. Ironically enough, there's no rules against in-park photo sessions there.

Personally I don't know why, but I'm guessing it's because they get less people than Disney World.