How did you get your little ones used to the huge characters and loud noises of Disney like the fireworks?
A lady in the office is really worried about this with her little ones having heard tales from a friend about her children who cried for two weeks solid everytime they went to a meal with characters or heard fireworks :(
Don't push. :) Even now with our little guy being a whopping seven, we still don't push. We show the character and let him decide. Most often he went (and still does) to the character but those times he didn't we were very accepting and matter of fact.
By the time you attend a character meal you will have an idea of whether the child will be ok with meeting one or not. If the child is not you can easily signal the character CM, and not have the character approach the table.
Fireworks... it's solely up to you and the child. If the child cannot tolerate loud noises then there is nothing to do for it other than leave. You could try viewing the fireworks from a distant area (a resort beach for MK for example) to give the child a chance to see how beautiful they are without the noise being quite as loud (but you will still hear them, of course).
I remember seeing a little girl with those big noise cancelling head phones...sure it didn't wipe out the fireworks totally, but it helped. She was all smiles.