Just because you bought the dress ahead of time, don't think you won't necessarily have to buy more... most of the rides and such dump out into souvenir shops, and all the princess dresses have accessories, tiaras, shoes, etc. You dodged a bullet getting the actual dress, but you might have to spring for these things. Expect to pay between 30-50 dollars for a souvenir, I believe the last time I went, the medium sized stuffed animals were 20-30. T-shirts run from 20+, hats 15+. I would highly recommend bringing a roll of quarters and pennies. Many places have penny presses, and you can distract the kid from souvenirs with the hunt for them.
If you are crafty, you can make mickey mouse ears at home with material, a headband, hot glue, and crafting foam. There are instructions all over for them, do a simple google search, these ears can get a heck of a lot fancier than the ones in the parks. The girls and I did them last year and every where we went people asked us where we bought them. I laughed because I made them from scrap jeans that didn't fit anymore, and rhinestones, making all 3 cheaper than one of the disney ones, and it took about 2 hours. Christmas ornaments are not cheap and they tend to be about 25+. If you're looking for the mickey mouse ear balls, skip them. There are instructions online on how to make them, and they look just as good as the real deal.
As far as OOP meals, quick service lunch and dinners run about 10-15 dollars, if you aren't trying too hard to find a good deal. That being said, Disney had *large* portion sizes. We used to buy 2 meals and split it with 3 people, and still had food left over. You might get away with not buying your daughter any food at all, it's entirely possible.
I would recommend springing for some sort of autograph thing, either a pillow case, autograph book (last choice,) picture frame, etc. Remember thick sharpies are best! This year I'm trying shrinky dinks for autographs. I get them signed by the characters, shrink them down, use them for christmas ornaments. I bought a bulk package of 50 pages for 25 off the interwebs, and this is enough to get signatures for all 3 girls.
YMMV, but I've found a trip to the barber shop on mainstreet first thing when we arrives is a great alternative to the BBB (bippity boppity botique.) You can get her hair cut for 15 dollars (on par for most hair cutting places outside of the dis) and she can get disney confetti put in her hair. They offer color gel as well for an additional 5 dollars, but I've never been impressed with it. Looks bad after a few hours. The confetti can stay in for days though, and when it starts looking sad, we stopped into the barber shop again and they sprinkled us up for free. Fair warning that it ends up *everywhere.* There's a reason crafters refer to confetti as a disease that you can't get rid of.

At least this stuff is large, as big as my pinkie nail.
I'm not sure about taxis, if you're staying on site, don't forget about Magical Express, you can get a free ride to and from the airport. I think I answered all the questions... if I missed anything lemme know
