Hi Natalie,
I have been where you are trying to stay on a program and eat at the theme park. My family went to Disney World one year and after walking around all day trying to "do the right thing", I let my husband talk me into eating a buffet meal at a German resturant. Needless to say I overate,since it was also my son's birthday we got a free cake. The cake was not very good, but I ate it anyway. I tell you about this experience to say this, eat a good breakfast before you go to the park, take a healthy snack to aviod eating the wrong things, eat chocolate in moderation, and drink plenty of water. Another thing is you will do alot of walking so you will probably break even. The most important thing to remember is to have fun and enjoy the time with family and friends without focusing on eating.
Have a great time!
Kim