Hi Judith,
This is a really tough call. When I plan our family's vacations, I try not to make table service reservations, or make one's that can easily be cancelled if our travel plans go awry.
The tough thing about Fantasmic! is that it only shows on select nights, and it really is a magical show that I recommend to everyone. How long are you staying? If you are able to push your reservation back an hour (or even 1/2) or better yet, make the reservation for a different night, I would suggest doing it.
I'm thinking it will take you about 2 hours from the time your flight lands for you to get to All Star Music Resort (ASM). Assuming your flight runs super smooth and your flight has no delays, that gets you to ASM around 3:00. You will then need to check in to your resort. I strongly recommend using
online check-in if it's available to save you some time. You probably won't really have time to go to your room once you've checked in; I would just hop on the first bus heading to Hollywood Studios.
Even if you choose to skip the Fantasmic! Dining Experience on this trip, there will many other amazing things for you to see and do on your first magical adventure to Disney. Keep in mind, you can still see Fantasmic without adding the dining option, you'll just have to stand in line about an hour before the show to get a good seat - maybe earlier if the park is very crowded.
Hope this helps. Have a magical vacation!