Hi Christine,
What a lovely treat for your niece, eating in the castle is pretty special!
I would recommend you take her to either breakfast or lunch as that is when the Princesses appear, at Dinner it's mainly just the Fairy Godmother. What I suggest you do is have a look at the menus and decide which you think your niece would prefer, breakfast can be found
HERE and lunch
HERE. If cost is a factor then breakfast is the cheapest option but this also makes it the most popular choice so harder to get reservations for.
At both breakfast and lunch Cinderella will meet you in the lobby for a photo and the Princesses will be in the restaurant to meet your niece for photos and autographs whilst you dine. There is also a very cute wishing ceremony which is performed at both!
Whichever you decide you must call for reservations at exactly 7am EST when you are either 90 days away (use the calculator at
www.pscalculator.net) from the date you require if you're staying off property or 90 days before your arrival date if you're staying at a Disney resort. Also try and be as flexible as possible with dates and times for the best chance to secure a reservation in the most popular restaurant in Disney.
Have a great trip,
Jo