Hi Melissa,
Our family loves Fantasmic. It is my oldest son's first request on every Disney trip. The show is held in a darkened ampitheatre. The approximately thirty minute show features music, water, lights laser and fireworks all centered around the battle between Disney good and evil. It features some loud bursts of sounds and lights and many of Disney's most famous characters are there to join in the fun.
We have been taking all three of our boys to this show since they were infants and have not had a problem. Although the show can be loud and the villains are present, the actual stage is across a small pond and is not as frightening. There are a lot of times throughout the show with upbeat Disney songs, princes and princesses, beautiful floats and more. Every child is different so if your child is frightened by bright lights or sound you may experience problems. You may want to consider sitting near the back or on an aisle so if your child becomes startled you can leave. There are personal video tapes of the show on
www.youtube.com. All you need to do is enter Fantasmic Hollywood Studios in the search field. Try viewing a portion to see if you think your child would be frightened, or even watch a portion with your child to see his/her reaction before you leave.
Hope you both love Fantasmic as much as we do!