With Disney's What will you Celebrate, she will either get free admission to the park or a gift card of equal value. You WILL need her Birth Cetificate to validate that.
We are going Next November and Celebrating my DS's 8th Birthday. I'm making him a special shirt and the rest of us will have shirts that say "Helping my Son/Big/Little Brother Celebrate 8"
His Birthday falls on Thanksgiving that year so it should be fun!
My DD got the gift card on her birthday this year, and had so much fun spending it!
If I'm not mistaken, that may end at the end of the year. I could be wrong. We did bring her birth certificate, as well as register online ahead of time.
I wouldn't overdo it for a 5 year old. It may be a long day, and they sometimes need some rest or downtime as well. My DD never would have made it to the fireworks at that age, but we always made our days early. Definitely if you can get a nap in, it would be fun! I'm not too familiar with the BBB - like if you take a nap, it will mess up the hair a little (I know they get it pretty well stiff, but may be uncomfortable to sleep in then?). I guess my DD isn't too girly, so she would hate BBB - but many girls love the girly stuff and the dresses! I saw one great trip report recently, and the DD dressed up so cute every day (my DD would never, sigh) and had crocks that she put coordinating bows on, so the shoes looked so cute and matching (same shoes every day, just different bows) but she was in comfortable shoes for her. That was a great idea - if you're wearing the princess gown, get matching bows for your shoes (could work with sneakers, using ribbon for shoe laces).
I don't think it's worth 2 table credits for lunch - you could do California Grill for the same credits, and we've had wonderful food there.
Definitely tell EVERYONE it's a birthday! For each ADR, mention it. Have it put on your resort reservation. Remind them as you are checking in, as they may have a button to give her then (even if it's early - let the whole trip be a celebration for her!). I think if the cast members know, it helps them, if they have an idea to help you celebrate. They can't do anything if they don't know! If you go in not expecting any freebies, just best wishes, then it's great!
It is expensive, and I wouldn't see it as worth it for my family, but the Disney florist does some gift stuff that they set up in your room. I saw on another trip report a family that did the one birthday celebration - it's I think 4 boxes of gifts from the characters plus streamers. It was a couple of hundred dollars or more I think, and you could buy things yourself and set it up while she's distracted or out of the room instead, but it looked cute. You can also order a special cake sent to the room.
You could also put together gift baskets, ship them down to yourself, then set them up the first day while she's out of the room (don't know if you have other children, but I'd do it for all of them). Maybe with a water bottle, tee shirt, light up toy or glow sticks, a little sweets, a small stuffed animal (my DD liked it when on vacation she'd get a new one and sleep with it). Something to start the celebration off, but not overdo it to be overwhelming.
Sorry to ramble, but I do love this stuff! Hope it's a magical trip and birthday!