If you have a PS prior to park opening, you show up at the park about 20-25 minutes before your PS and a cast member will direct you to a line of others waiting to get in too. You'll be amazed at how many people are there that early.
Chances are you will not fininsh eating before the park opens, especially at CRT with the princesses and all, but if you did, you could only stay around the castle. They would have ropes up to the "lands" and you would have to wait for the ropes to drop when the park officially opens.
Have a great time!