We always do Liberty Tree Tavern before special events at the MK. It's typical Thanksgiving fare (turkey, mashed potatoes, mac and cheese, etc.) and quite good. It's also a character dinner with Minnie, Goofy, Chip, and Dale. Usually they're dressed in Colonial costume, but on MNSSHP nights they add "halloween costumes" to their outfits.
There are no princesses at the evening meal at CRT--they're only there for breakfast and lunch. You may try to do a late lunch there instead of dinner if princesses are important.
We had a great time at CP last year before MNSSHP. Their last seating was fairly early due to the party. We ended up being one of the last dining parties still hanging around with quite a few of the characters. Tigger and Piglet came to our table several times, and our kids were delighted. Also, it's a buffet with a fairly wide selection of dishes, so I'd bet your niece could find something she likes.