Hello, Maria!
You made two great choices for meals at Disney's Hollywood Studios!
Now, you don't need make a reservation to see Fantasmic!; it's included with your park admission. That being said, however, folks often get to the venue an hour early, especially if it's a peak season, just to be sure to get a seat. While I'm not certain of the date of your Spring Break trip, I can tell you that many dates in April actually have two Fantasmic! shows, one at 8:30 and another at 10, for example. If your dinner reservations are for 7 p.m., you might not be able to get a seat, or, perhaps, even get in to see the show at 8:30. For a 10:00 show, you should be just fine. Check
Disney's Hollywood Studios April Calendar for more information (if your Spring Break is in a different month, you can change the month view easily from this site as well.)
If you'd consider trying a different restaurant, at an earlier hour, for dinner,
consider booking a Fantasmic! Dining Package instead. While you'll still need to arrive at Hollywood Hills Amphitheater 30-45 minutes prior to showtime, you'll be escorted to a VIP seating area for the show. You can also call (407) WDW-DINE for more information. We had a great meal at Mama Melrose this past fall and sat ten rows from the water line at Fantasmic! that night.
If you have any additional questions about your upcoming Spring Break in Walt Disney World, please send them our way! We love to help!