No dress code for 1900 Park Fare.
We took DD6 last year after she had a full make over at BBB, so she was all decked out. You see alot of girls all dressed up and then everyone else is a mix of normal park attire (nothing special).
I have no plan on going to 1900 Park Fare again, but booked Crystal Palace again this year for breakfast. We enjoyed the food, characters and getting in to the park early!!!!!!!!!!!!