April,
Thanks for your question, and congratulations on that wonderful milestone!
You certainly are welcome to wear more formal attire into the parks for some photographs. While it may be a bit warm to wear on an everyday visit, I have seen people in all sorts of outfits. Disney does post some rules about what qualifies as
appropriate attire for the parks, but most of what is prohibited includes adults dressing like Disney characters, or clothing that might be considered offensive.
A personal tip would be to consider renting a locker for the day.
Locker Rentals are available in each park, and you could pack a comfortable change of clothes for after your pictures have been taken (or before, dependent upon your plans). You could then change in to more touring friendly attire, stow your formal gear in the locker, and enjoy the rest of the day at the parks without having to return to your resort.
Another idea for you to consider is the
Disney Fine Art Photography Service. We did one of these photo sessions many years ago and the photos are to this day some of our most prized possessions. You can call (407) 934-4004 to get more information and to find out what the cost might be, based on what moments you are looking to capture.
Have a Magical Vacation!
Doug