Hi Cindy,
What a great way to celebrate your little princess's birthday!!
I checked the calendar for Hollywood Studios, and it looks like you can see Fantasmic! on the following dates:
Thursday, Sept. 23 - 8:00 PM
Sunday, Sept. 26 - 8:00 PM
Schedules are subject to change, so I recommend checking the
calendar again before you leave. Also, be sure to check the daily entertainment schedule when you arrive at the park.
Hope this helps. Have a magical vacation!