Hi, I have a noise sensitive almost 5yo DS, but he's seen all the fireworks several times (except Fantasmic).
You can see the fireworks from Illuminations, but not the music or lower level laser show, from the bridge between Boardwalk and Beach/Yacht Club. It's fun to walk around the Boardwalk, see the performers and grab dessert, a good distraction for the small ones before the show's on.
Also, my son has learned to wear hearing protection, the big ear muff type that adults in loud work evironments use. He now carries them with him on our trips, wears them on the planes and even for the parades or shows if he thinks they're too loud. I don't know if your child might consider doing this. They can still hear most of what's going on, it just muffles it enough to be tolerable. When he was little, since we usually went in the Fall, we'd be sure he wore a thick sweatshirt with a hood, and put it on for the loud parts, with our hands over the sweatshirt and covering his ears. That seemed to help too. Of course, nestling in Mom's or Dad's lap goes a long way.
Whatever you do, have fun!