There is a set menu for the Hoop Dee Doo Review (fried chicken, ribs, cowboy beans, etc.). However, there is a bit of flexibility in this.
We first enjoyed the HDDR when my twins were about four years old and my picky eater daughter was in an extra-super-double-plus picky phase where pretty much all she would eat was mac 'n cheese. When we were seated the show, we explained this to our server. She was able to bring out a big bowl of mac 'n cheese, enough that the entire family could have some. Similarly, I dined at the HDDR with a friend who does not eat meat, but does eat fish. They brought her out a lovely piece of grilled salmon instead of the chicken and ribs. The HDDR shares a kitchen with the Trail's End restaurant next door. It is fairly easy for them to bring you out anything that is served at Trail's End, which has a much larger menu that the HDDR. Just ask nicely and they're usually able to accommodate.
If you have allergies or food-related medical needs (rather than just a picky eater), then you should inform the restaurant in advance. Most special diets can be accommodated at Walt Disney World. There is
an entire department at Disney Dining dedicated to helping guests with
food allergies, food intolerances, and
religious-based food requirements. Just send an email describing
your situation (at least 14 days in advance of your trip) to
SpecialDiets@DisneyWorld.com.
A cast member will get back to you with the information you need to
make sure that you have a safe, and delicious, visit to WDW.
Have a great trip!