They’re about average for a typical fireworks show – except for the holidays (for something like Illuminations) where it really can rattle your innards out.
What’s nice about Disney fireworks is that they use compressed air to launch the fireworks, so the “boom” of the launcher isn’t as loud as other fireworks displays. It is still loud enough that if the weather is just right, Wishes will rattle the windows on our house about a mile and a half away.
That said, when our daughter was two, we were able to take her to fireworks and have her sleep through them. It really depends on the kid. The loudest place in MK to watch them is in Fantasyland, where they are right over your head and you’re pretty sure they’re going to fall on you. (Hey, they have sprinklers on the roofs of the buildings for a reason!) The quietest place in MK is either at the far end of Main Street or a little distance back in Frontierland. For Illuminations, what you’ll want to make sure is that there’s no building directly behind you that the sound will bounce back on.
Illuminations is a good way to test a child with fireworks because you have about 2-3 minutes of moderate fireworks near the beginning, and then you’ve got the globe part of the show in the middle. If your child is freaking out after the initial fireworks, you can use the globe part of the show to move back from the lagoon or start making your way to the exit.