How about getting her a rolling suitcase (with the extendable handle)? She'll have her own carry-on bag. Give her her own refillable stainless steel water bottle (you can get a Disney-themed one on line or from the Disney store, or one with her name on it from LL Bean) - you can't take filled water bottles through security, and airport purchases after security are SO expensive. Get one with a loop or hook (or buy a carabiner) so it can be hooked to her backpack. You can fill them everywhere and that's so much easier than waiting in line for purchased drinks at the park.
Check the travel section of the supermarket for little bottles of shampoo and mini tubes of toothpaste or a travel size mini toothbrush. Those are fun. Or, buy a set of small 3 ounce bottles from any department store or a place like Bed Bath and Beyond for her own products. Don't get into the security reasons for 3 ounces of less - just that we don't travel with a ton of stuff. You could consider a neck pillow too - she may be too excited to sleep on the plane but it's good during other travel times - in the car on the way back to the hotel or rental house, etc.
You can take your own food onto the plane and into the park, so how about one of those sectioned plastic lunch containers for sorting food & snacks. I'm sure you'll want to get meals at the park, but a pack of snacks is a lifesaver while waiting in lines! You can also take some things in ziploc bags and refill as you go.
I've seen a lot of kids with those autograph books for when they meet a character - depending on how she feels about the characters, it can be fun. It can also be a big crowd to wait in instead of going on rides too, but it's a possible gift. And take pictures!!!
You might feel more secure if you have an ID bracelet made for her - I'm sure they have kid safety ones, but we all have Road ID ones for the runners in our family - anyway, you can print the child's name, the parents' cell phone numbers, any medical or allergy info, etc., in case she gets lost. You don't need to give her that as a gift, but it's a good purchase.
Some small hand-held games for the plane, for the waiting (airport, park) and the hotel are just great. Go to the "travel games" section of any big toy store - they have everything from Yahtzee to Travel Bingo to Connect 4. We traveled when our son was little with a non-electronic Connect 4 game - great for when devices have to be turned off. I'm sure you have, or can get, some cell phone game apps appropriate for her, but you'll want something for when the cell phones have to be turned off or for when batteries are running low.
I agree with you about not surprising her on the day of the trip! Too much anxiety for all! Give her time to ask questions. Get a book about air travel from the library, and help select the attractions at the park(s) that you will visit. Please understand that you cannot visit every ride! And recognize that there is a TON of walking so plan on really excellent walking shoes/sneakers for everyone. You'll see older kids in strollers too - don't be afraid of that. And think about leaving mid-afternoon and going back to the hotel for some pool time. It's okay to not have every minute in the park.
One trick I've seen in the "unofficial" guides and which worked well for us is to enter the park of your choice (Magic Kingdom, Epcot, etc.) and go directly to the REAR of the park. I know, I know - you'll have to walk through and past so many cool things. But the crowds are all at the entrance and moving toward the back. Do the opposite. Moreover, when everyone is cranky and tired, you'll have worked your way back to the exit/entrance and will be able to leave without trudging all the way across the park.
Be aware that some rides have height requirements and some are intense or scary. Take some video tours or look at pictures in the guidebooks with her, and let her know ahead of time which attractions are off the table. She can pick from what's left - and there is plenty. You can help her make a priority list (that's a good airplane activity too) and then sit with a park map and some removable stickers to plan a route. It's actually educational to teach map-reading skills (no matter how basic) and you'll have a rudimentary plan of attack.
Also plan some sit-down activities at the parks for aching feet - snack time, longer rides, just sitting on the grass or a bench. Help her set a budget for snacks and for souvenirs too. No souvenirs until the 2nd day at least, and only on the way home so you don't have to carry everything around all day.
If you think she's nervous about the flight, go ahead and plan a trip to the airport first, just to look around. Show her the security lines and explain that you have to stay together, show her baggage claim so she understands what happens to the suitcases vs. the carry-ons, etc.
And of course, take gum or something chewable on the plane for ear pressure. If you've gone into the mountains in Maine and she's felt her ears pop, that's a good start. The plane engine is noisy too, so a set of earplugs can be helpful. Also talk about the fun of people-watching and actually talking to strangers in a safe way so it's not all about having her nose in a game. We took small sized puzzle books with word matches or mazes, that sort of thing - then you and she can play together. Take some mechanical pencils with erasers so you aren't worrying about finding a sharpener. Be careful with markers - if the caps come off and get all over your other stuff, that's a pain.
If you are staying in a hotel, get a small refrigerator for things like fruit and yogurt and string cheese. Take some plastic silverware. After you arrive, buy a paring knife (which you will leave behind and not try to take on the plane) and a small bottle of dish detergent for washing your plastic containers out. Get an extra towel or 2 from housekeeping to use as a "dish drainer".
Take a couple of packets of travel woolite or something similar for rinsing out bathing suits (if there's a hotel pool), and maybe a travel clothesline (some hotels have them, some don't). And public bathrooms often have really awful toilet paper, so get a few of those travel wipes (I think Cottonelle makes some) - she'll feel cleaner!
Have her put a change of underwear and a clean shirt in her own carry-on suitcase. You don't want her to freak out about lost luggage in advance, but you really don't want her to panic if it happens. Remind her she's in the US and there are stores and drug stores and restaurants there, just like at home!
Hope you have a great time - we had several wonderful trips with our son and my step kids. Great memories.