3. As others have said, just call Disney Dining and have them add the birthday to the reservation. Have her wear the pin and you'll be shocked at how many 'happy birthday' greetings she gets, and how many cupcakes from restaurants. Call the BBB and have them note the birthday on that reservation as well. She'll get a "birthday princess" sash as well as the standard BBB one.
4. I would buy the ears on property. You may find that you'd rather have classic ones than the birthday ones for the first trip. It's just as magical to pick them out together and have her name embroidered on them as it would be to have them pre ordered.
6. No one can know how much for souvenirs is in your budget. We like to pick a 'per day' amount that we're comfortable with and use that as a guideline. That keeps us from overindulging DD, and we have NEVER actually spent the whole amount budgeted. It's a nice bonus to have extra money left over when you get home! If your kids are into stuffed animals or pins or whatever, maybe agree to buy them one per day. It's also nice to get them some Disney gift cards with a small amount on them and let them have some of their own money to spend. DD6 always saves up in advance of the trip to have spending money, but since your trip is a surprise, maybe ask the grandparents to give you some gift cards to surprise them with on the trip?
7. If your TA booked PP+ through Disney rather than the Photopass website (and I'm assuming she did) then the PP+ voucher will be in the Magical Extras booklet-commonly called Mickey Mail on the DIS-along with the coupons for the arcade, Planet Hollywood, mini golf, etc. DO NOT FORGET THAT VOUCHER! Take that voucher to any PP location (there's one at some of the deluxe resorts-we used the one at the GF since we had a breakfast ADR there on our first day-and one at every park). At the PP location they will give you a CD case and the red PP+ card on lanyard. The CD is very important as the code you'll need to order the photo CD once you've edited your vacation pics is on the inside cover. When using the PP+ card for ride photos, note the pic # and then go to the desk where you purchase them, tell them the photo # and have them swipe your card. The ride photo will be added to your PP card (no hard copies). For restaurant photos (and BBB) you'll get both hard and digital copies of the pictures.
8. My husband carries the backpack full of ponchos/extra clothes/camera gear and I carry a really small cross body purse holding the cards, wallet, etc. We take both on rides with us. My DH is a big guy (6'2" 250 lbs) and he's never had a problem getting himself and the bag into the seats comfortably.
Have a great time, and thanks to your family for your service.