Hi John!
One of the best things about Walt Disney World is that theme park admission for children under 3 is completely free. She will not require an armband for admission, just adult supervision. In addition, children under 3 are free when dining at character and other buffets with a paying adult, so keep that in mind as you plan your family's vacation. A la carte menu items or kids menu choices are not free, however.
If she is accompanying your parents to
Extra Magic Hours and they are staying at a
Disney Resort with valid park admission, she will be able to enter the parks with her. However if other members of your family are aged 3 and up, they will not be able to enter the parks during Extra Magic Hours without a valid Disney Resort ID and valid park admission.
I hope that helps you plan and that you and your family have a wonderful vacation full of Disney magic! Thanks for visiting the panel and if you have any other questions, be sure to come back and ask. We're happy to help!