Oh, the magic ahead for you! While we couldn't swing a Disney vacation until our DDs were 17 and 19, we travelled with our 3 grandchildren when they were between 2 and 4 yrs old. Don't expect your DD to remember the trip, but you certainly will.
One of the things your DD may not be prepared for is the noise. Things are really LOUD. We found our little ones often covering their ears of trying to duck their heads. If she is sensitive to sound, some little earbuds or headphones may help a bit. It is funny, but even a soft stretchy hairband can give some children the sense their ears are being "protected" even though the decibel level is not appreciably decreased.
We have talked often about what were our most successful strategies that trip. Strollers were close to the top of the list. None of the children were prone to meltdowns during those years, but we all still believe the strollers were one of the biggest factors in avoiding them. My heart breaks when I see young ones being (literally) dragged along through the heat and crowds.... and we rested OUR feet while they played in the wonderful areas designed for their age or cooled off in fountains. They walked a good bit, but rode at least 50% of the distance. Important to note that we often walk over 10 miles during an Epcot day and not many 4 yr olds are ready for that.
Snack bags were a big deal. We packed them every day. Each child had his/her own little Disney lunch bag, with juice, fruit, cheese and crackers, and even sandwiches. These attached to their strollers and they could snack when the mood struck them, as long as they were riding. Likewise, they always had water at hand. Saved us a small fortune and the kids never went into that zone of being just a bit too hungry or thirsty to be pleasant. We seldom ordered any kids meals, but just let them graze on whatever appealed from the adult plates. No waste, no pressure to eat what was ordered. Ice cream and popcorn buckets were special treats.
BTW, we stay off site, and had cars. We bought very inexpensive strollers in Wal-Mart, expecting to leave them behind, but my DDs liked them so much that they brought them home on the plane.
Touring plan - we had one, but it was a guideline, not a schedule. The hard and fast rule was that if any part of our party had reached their limit, they could go or be taken back to our vacation house. WDW is a super-stimulating place for all ages, and it can get overwhelming. When it does, it is not fun anymore. No one reached that point, though.
We still congratulate ourselves on the fact that we never carried a child, had to deal with a tantrum, or even an upset tummy. The kids slept soundly through all but the
MVMCP fireworks, went to bed easily, and got up bright-eyed each day...and so did we. Granted, we had 3 youngsters, with 4 parents and 2 grandparents, so there were lots of hands on deck. But we also had an 89 yr old great-grandmother along, who required almost as much supervision on her first ever visit!
Expect the best, be flexible, and when something isn't working for you, do something else. Some time at the pool is often as good as a nap. Don't tolerate behaviour on vacation that isn't acceptable at home, and do your best to avoid the triggers, usually hunger, thirst, fatigue, and over-stimulation... ahem... again, important for ALL ages.
Really important note - despite getting used to things, children's reactions to situations can drastically change from day to day at that age. A child who embraces every character one day, may hang back the next, or even cry. Same with rides and attractions. Children's perception is very different from ours and anything from a movie scene to a dream can cast an experience in a new light, and turn something simple into the stuff of nightmares. If this happens, don't waste time coaxing... redirect and move on. This is your vacation, plenty of time for life lessons later. Share the joy and move away from things that don't provide it. It's one of our few opportunities to do this!