My son (now 6) is SO not a fan of loud sounds - he gets so overwhelmed by the BOOMs of the fireworks that he has a hard time enjoying watching them. One thing that might help, if your kids aren't at all familiar with fireworks, is to find a video on
YouTube or elsewhere on the web and show it to them. So they know that the pretty lights and the sounds go together and they know that every loud boom means another burst of color in the sky - sort of preparing them a bit for the noise as well as showing them what to look forward to...
I'd also suggest watching from further back instead of right in the middle of the fireworks. (My son agreed to watch Wishes from just behind the Castle on our last trip. We didn't realize we'd literally be in the middle of where the fireworks were set off. Poor kid was miserable and didn't even watch. He just covered his ears and curled up in my arms the whole time.)