Hi there...
We had a 9:15am reservation and the World Showcases did not open till 10am...
It was no problem at all, you just tell them at the gate you are having breakfast and they check you off on a clipboard and send you on your way,
It is actually a great time to take photos of the showcases when no one is around..
We arrived early and they gave us a vibrating coaster to walk around with and buzzed us when the table was ready.
Breakfast was great by the way, with muffins and fresh fruit, french toast sticks, scrambled eggs and these great onion-cheesy potaotes.
SLeeping Beauty, Belle, Mary Poppins, Jasmine and Mulan were there my morning and we had a great time.
Hope this helps