We took a 5yo to the Cinderella dinner at the Grand Floridian. She was dolled up in her princess outfit and had an absolute ball. There were many younger than her, also in tiara's feeling like the princesses that they are. The Grand Floridian is also a beautiful place so leave yourself some time to stroll the resort before or after the meal.
As for enough for one day, you could spend some morning time in the park. Enjoy your resort/rest in the afternoon then have a great dinner. It is possible to see Wishes and the Electric Water parade from some outdoor locations at the Grand Floridian too. Another nice treat for her if she can stay awake.
My grammy took me on my first trip to WDW too. I was 12, but the magic hasn't left me since and I am a whole lot older than 12!