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Good morning. A copy of her birth certificate should be sufficient, if asked, for her to travel with you and board the plane. You can bring the expired passport but typically the airline will require a birth certificate. This really should not be an issue since you are traveling within the US but you can always call and confirm with AA that the birth certificate is all that is needed, in the event that you are asked.
Here is a link to their website, evidencing the same.
I just wanted to follow up and see if you had any other questions or needed me to clarify something. I am here to help, so please let me know. Thanks!