Under 3 is free at places that have all you can eat buffet, pre-plated meals etc....so your child should be free at PSB. If it were a regular restaurant with a menu, then you would pay for whatever you order for her.
The princesses are subject to change but usually Ariel is the greeter at lunch and dinner with Belle (in her blue peasant dress), snow white, Sleeping Beauty, Mulan and Jasmine circulating but you won't know for sure till you get there.
Have fun!