Hi there, Iris!
You're really going to enjoy dining inside of Cinderella Castle, especially at night! The entrance to Cinderella's Royal Table is actually on the Fantasyland side of the Castle, so you'll still be able to exit the restaurant if the fireworks are going on when you're finished with dinner.
I think it's really fun to watch the fireworks from different spots around the Park because it gives a completely different perspective every time. You really can't see the fireworks very well from your table inside
Cinderella's Royal Table though. There are gorgeous stained glass windows lining the entire restaurant, and even if you're lucky enough to be seated at a table near the windows, the fireworks are way above the Castle. You might be able to see some of them, but I recommend trying to plan another night at Magic Kingdom Park to view them from outside.
You'll be visited by classic Disney Princesses throughout your meal, even though it's a late one. The Princesses will come to your table to visit, pose for photos and sign autographs. It's such a magical one-of-a-kind experience that will create such wonderful memories!
I sure hope this helps, Iris! Please let us know if we can help with any other questions you have.
Wishing you a Pixie Dusted vacation!
~Jackie