Hi Maria,
You are smart to try to book this so early.
I have some good news and some not-so-good news. The not-so-good news is that if you are not having success booking the dinner reservation online, that is most likely because it has already sold out. This is one of the most popular dinner reservations and the Dining Reservations people tell me that they sell out very quickly. To be sure, call 407-WDW-DINE to ask them personally.
The good news:
Fantasmic! is a free show, included in your admission to
Disney's Hollywood Studios. So you can eat at any restaurant first, or bring food from anywhere in the park into this outdoor theater area (food is available in the Fantasmic area too).
You do not wait in line for Fantasmic! - when you enter the outdoor
theater area you can find a seat anywhere and sit there while you wait.People generally find their seats early for this show, and the seating is often filled to capacity. So you are urged to get your seat as early as possible. Once cast members determine that the seating areas are completely full, they will not let anyone else in.
Maria, I hope you have a magical family holiday!