Any suggestions where the best place to have it is? I would love somewhere that will make it special for her.
It depends on what mean by "make it special for her"...are you asking if there are any places that will do something special for birthday guests without you having to do/pay for anything OR are you asking about places that do something special if you pay for it OR are you asking about places that seem unique to Disney?
At most restaurants, you can order a small round white or chocolate cake from the podium. They're about 6-inches and then run about $25 (I think). I don't think they can be customized in any way (i.e., no writing on the frosting).
If you want to splurge, you can order a custom cake and have it delivered to the restaurant (not all restaurants are eligible). You can find more information about options on the Disney Food Blog.
http://www.disneyfoodblog.com/disney-world-cakes-faq/
There is also a cute Cinderella's Slipper dessert that's available at CRT and the restaurants at the GF. It's cute looking (I've never had it, but I've seen one.)
http://www.disneyfoodblog.com/2011/06/30/celebrate-with-a-chocolate-cinderella-slipper/
In terms of "free" stuff, it really depends on the restaurant, the server, etc. Most times, the server will wish the birthday person a "happy birthday" and that's it. Sometimes they'll give the birthday person a free dessert (usually a cupcake). You really won't know what will be done (or if anything will be done) until after you get there. So it's best to not expect anything. Overall, I've had better "luck" at the Signature restaurants during non-peak dining hours. Just a note, but the only place where I've ever heard them sing Happy Birthday was Via Napoli; I've never heard it at a Signature.