This is the answer I got from Disney when I emailed the same question. " You may purchase your tickets according to the age of your child on your arrival date. If your daughter is still 2 years old on your arrival date, you will not need to purchase a ticket for her.
We look forward to your arrival."
I plan to bring a copy of this letter, a copy of her birth cerfiticate, and something that says when we checked in. That's probably not nessasary but she won't be in a stroller and is very tall so I know I'll be asked how old she is.