DH & I have always said that 3-4 years old is such a magical time to go to WDW! You will greatly enjoy her wonder!
Plane - Talk to her dr - She likely will not have any problems (our granddaughter did not), but what to do if the air pressure causes her ear pain. I think putting paper cups over her ears will release the pressure, but I'm not sure. Maybe he/she will want to give her a med to clear the fluid from her ears before she flies. As an adult, I have to do this. I have seen children screaming with pain on planes. Also, I'm sure not to be asleep & to have my head upright during takeoff & landing (pressure changes). That allows the extra fluid in my ears to properly drain. Once I was asleep during landing & had an ear ache for 2 weeks!
WDW - Best advice anyone can give you is to take every day at her pace, not yours. A hot, worn-out child is an unhappy child & creates unhappy parents. Get her a stroller to save those little short legs! Plan on mornings & lunch at the parks (when it's cooler & less crowded), then back to the room for a nap & swim, whichever comes first. If more park time is wanted, go in late afternoon or evenings when others have worn themselves out & left. Once we were at MK late & no one was on the Peter Pan ride (DGDs favorite). The CMs let us just stay in a flying car & we rode it endlessly!
Ft. Wilderness Campgrounds - Our granddaughter loved the pony rides, petting zoo, playground & renting a boat. She had a favorite pony (Cinnamon) that had to be visited often & she waited until he was available for her ride! She also enjoyed renting a golf cart & riding all around. Trails End restaurant is a good place for lunch too. Actually, we liked staying in the Ft. W. cabins with a golf cart for getting around. Quick access to MK, her favorite park & then all of the other activities too.
Outfit her appropriately - sun hat, sun screen, sunglasses, cool clothes, long sleeves for evening & early morning.
Don't weigh yourself down with lots of purse/bag items while at the park. It restricts both of you. You can get water & snacks everywhere. Restrooms for washups. Just need clothing mentioned above, tics, money, ID cards.
"Label" her & you - On your pc on both sides of a business card, list your names, addresses, your travel cell phone #, home emergency contact person #, health info, health ins info, travel ins info, itinerary including hotel, airline name, dates, & flight #s. This is important for you also, in case you are in an accident & can't speak. We used to securely pin this info to our children & DGD's clothing so it didn't end up on the ground. Should be on both of you at all times.
As you enter the parks, advise her what to do if she gets separated from you at the park - go to a CM, & point out their uniforms, & they are in stores & food places.
Take lots of pictures &/or videos. We so enjoy those now - kids grown & DGD in college!
Enjoy
