Bruce,
No only can she wear her birthday celebration button each day of her trip, she
should wear it every day!
When my family travels to Walt Disney World it is typically a birthday celebration for my two girls. Their birthdays are in the middle of the school year, so we are never there on the exact dates of their birthdays, but that is what we are there to celebrate. So we pick a different day during that trip for each of them as their own personal celebration day. But they wear their buttons all trip long. They love the attention, especially the friendly greetings from the cast members. Remember to also note that celebration on your dining and resort reservations.
BUTTON TIP: When you are requesting the button from your resort's desk, or
Guest Relations in the parks, ask them after they write her name on the button if they could put a piece of tape over that. The marker writing has a tendency to rub off, and you would not want that!
If you are picking up the button from a theme park, make sure you go to Guest Relations after you have entered the park. The lines are typically shorter. They are located at the following places;
Have a Magical Vacation!