There's a whole different pricing tier for the private sessions, but you'll often times find them in the resort in the evenings taking pictures of guests like they do in the parks.
When you make a reservation, you can inquire about the costs. It's reasonably priced for the service that you get. And you do get much better qualified photographers for these sessions. They have to go through a second portfolio interview to get into the resort portraits.
If you would like a couple of nice family photos, in a studio like setting, you may use the photographers at the Studios for the BBB pictures. Princesses have higher priority, but late in the afternoon, it's usually quiet for the photographer to grab some good shots. Many people use it during the Halloween parties, they keep the studio open for a few hrs into the party.