Hi, Jennifer!
What a special treat for your princess! Dining at Cinderella Castle is an experience to treasure.
I suggest booking an early breakfast reservation. There is something so special about entering the Magic Kingdom early in the morning. If you are early risers, plan for a breakfast around 8:00 a.m. You can enter the park before the gates officially open and have Main Street USA practically to yourselves allowing for some amazing photo opportunities.
Once your breakfast is done, you can head over to the New Fantasyland, and by getting this early start you will find shorter lines and smaller crowds.
If you decide to go with dinner, I would opt to eat dinner on the early side. You do not want your princess to be too tired when she meets Cinderella and her royal friends, so plan on booking a reasonable dinner time.
Whichever meal you choose, reserve this as soon as you hit your 180 day booking window. Dining at Cinderella's Royal Table is one of the most popular dining experiences at Walt Disney World and tables fill up quickly! If your daughter chooses, she can even dress up in her finest princess gown, but I would bring a change of clothes in case she wants to put on something a bit more comfortable.
Good luck choosing your dining locations, and
let me know if you need any other suggestions.