Have fun! We did this in February. First, 11:30am is the first lunch sitting. We had an 11:30 reservation too and it was perfect, I would do it at that time again. We got there about 11am and checked in, went to walk around, came back at 11:30 am and got to go directly downstairs. This also put us in the third table in the order the Princesses, perfect for my 3.5 DD that would not eat until they were all done visiting us!
You see Ariel at the bottom of the stairs, get your picture taken, then they sit you at your table. Food starts flowing immediately. Once tables are full the family of the meal was chosen and got to read the first Princesses arrival. Snow White was first, she started at the tables, and went table by table. It seems like maybe there was 5-10 minutes between each arrival. DD barely got seated before the next one came out. We finally had to get her to take a bite between each, so she would eat!
We saw Snow White, Belle, Cinderell, and Sleeping Beauty. In that order. When we left people were still mingling at their tables, so I am sure the next sitting was slow going, that is why I would do the 11:30am again!
Have fun! Oh, and mid lunch they came to sell you a package of the picture they took with Ariel. We bought it. DD was dressed as Belle, so on the 4x6 pics they had Ariel's signature in one corner and Belle in the other!