Hi Anna,
A character meal with the Disney Princesses sounds like a lovely way to celebrate your daughters birthdays during their very first visit to Walt Disney World!
You have two main choices:-
1/
Cinderella's Royal Table at the Magic Kingdom
2/
Akershus Royal Banquet Hall in the Norway Pavilion at Epcot
The Disney Princesses are present during breakfast, lunch and dinner at both of these restaurants and the price per person ranges from $36.00 - $59.99.
Although both of these character meals are extremely popular, you never know your luck. I would recommend that you search the online dining reservation system for availability during the dates of your stay as soon as possible
HERE.
If there is no availability at either of these restaurants, it may be worth considering dinner at
1900 Park Fare at Disney's Grand Floridian Resort & Spa where you can see Cinderella, Drizella, Lady Tremaine, Anastasia & Prince Charming.
I really hope that you manage to secure a dining reservation and wish you a magical trip!