Aw! That is so sweet!!! It is a wonderful thing you are doing!
My suggestion:
If she knows she is going to Disney tell her that everyone gets to choose a meal and let her choose the place she most wants to dine at! Tell her it doesnt matter where she wants to dine, but to pick somewhere that will be magical to her! I think she will look forward to this meal the most THEN you hit her with your question, and I really think it would be beautiful for her!
I am 20, and I would personally suggest either CRT (but characters move FAST and there may seem to never be a good time to ask) or I would do California Grill around Wishes time, so you can have the beautiful fireworks and enjoy the evening.
I think 50s would be decent but at 16, this wouldnt be ideal (for me at least). 50s is its own experience (with the waiters and waitresses in character...ect) it wouldnt have the wow factor to me.
Another suggestion:
(I have personally NEVER done this..so Im not sure what the atmosphere is like)
BUT you could try doing Wishes Dessert Party and asking her during the fireworks. Im a sucker for the beautiful fireworks and the Magic Kingdom atmosphere..I get very emotion! haha!

Somewhere with the beautiful firework setting would be great!
If you choose CRT: I know they have a special package for engagements, and the woman gets a glass slipper (I think!), you could do something like this but gear it towards adoption..and not marriage, obviously. BUT then again, this may be cheesy for her, and Im all about cheesy (it is alll about a MOUSE!

)!!!!!
Good luck! Let up know what happens! Either way, dont stress about it, it will be a BEAUTIFUL day!!!