Hi there, Barbara!
September is a great time to visit Walt Disney World, I'd recommend arriving after Labor Day. The last two weeks in September are substantially lower in terms of crowd level than much of the rest of the year. And make sure to ask for your complimentary "First Visit" button on check-in!
For your daughter, it's not the characters that will make the Parks incredible; it's the experiences that you'll have as a family and the memories that you'll make together. There is absolutely no reason for your little one to engage with any characters unless she expresses the desire to do so. The magic, for her, will come from the rides, the parades, the shows, and the fact that she is with her family, her favorite people in the World.
She can always have some fun playing with the Cast Members, who are, more often than not, available for conversation and dialogue - particularly those whose job it is to entertain the children before parades. This might ease her in to the idea of talking to Disney-specific folks, and the Princesses could be a nice follow-through.
To make potential meet-and-greet that much more enticing, consider getting her a makeover at the
Bibbiti Bobbiti Boutique. She can have her hair done and be costumed by a "Fairy Godmother," who will make sure to remind her of how much like a princess she looks. From there, head over to one of the Princess dining experiences:
Akershus Royal Banquet Hall in Epcot is an excellent one - and she might very well be dazzled by the attention she'll receive by the characters.
If you'd like, drop me a line and let me know some of her favorites; I'd be glad to create a few specific suggestions to help your family maximize the magic on your first vacation.
See you later, celebrator,
-Deborah B.