When my youngest DD was 5, we were flying to Disney - DD was so excited - she kept saying she hoped the plane would pass over her school so she could wave to all her friends. (She had flown before, when she was almost 3 and really slept through the flight.) But, she couldn't wait for December to hurry up and come!
Well, we got on the plane and as the plane just starts to back up, she decides she doesn't like this afterall. She wants to get off. I hugged her, and got out her stuffed puppy and we were hugging and I was talking to her about Disney. When the engines really started up so we could start taking off, she was terrified and crying. This started all the kids around us crying. The look of fear on her face... I was thinking that this would be a LONG flight to Orlando. Fortunately, once the plane was in the air, she quieted down and was OK. She got a little scared for landing, but we played a game of looking for the castle. (And I didn't have the heart to tell her we'd have to fly again to get home)
The next time we flew, DH was with us and he made everything OK.
Since then, she's flown quite a few times, including a trip to Las Vegas, and has been fine. Coloring books and hidden picture books work best with her. And, I always bring some snacks that she doesn't always get when we're home. Thankfully, our flight to Orlando is less than 1.5 hrs.
Just be prepared for anything! (I had no idea she would be afraid to fly) Have a great trip!