Best to ask what Princess(es) they want to see the most and plan around that. Different meals have different princesses, and some like Tiana and Elsa never do them. Here's a general overview:
Bon Voyage [breakfast only]: Rapunzel, Ariel + their boyfriends
1900 Park Fare [dinner only]: Cinderella characters only (the prince, stepfamily and herself)
Artist Point [dinner only]: Snow White characters only (Evil Queen, Grumpy, Dopey, and Snow)
Cinderella's Royal Table [3 meals a day]: Cinderella guaranteed, plus four others like Ariel, Jasmine, Snow White and Aurora. If Belle is there she's in her blue village dress as her yellow gown is too big to squeeze between the tables (yes, really!). Rapunzel on rare occasions.
Akershus [3 meals a day]: 5 princesses total, but other than Belle in the lobby it's the least consistent. Most of the time it's Aurora, Ariel, Cinderella and Snow White going from table to table, but you could also see Jasmine, Mary Poppins, Mulan or Alice in Wonderland. The reason is that they sometimes pull CMs who do other meet and greets elsewhere in World Showcase, so that's why there's a wider range of characters who may show up.
Grand Floridian Princess Tea [select mornings]: Aurora only, and it's over $300 for 1 adult and 1 child.
Breakfast at 1900 Park Fare does not have any princesses, but is still fun with Mary Poppins, Alice and the Mad Hatter, Pooh and Tigger.