Hi Angela,
You're going to have so much fun at
Mickey's Very Merry Christmas Party - it's one of my family's favourite
Special Events at the Walt Disney World Resort.
I can tell you that last year, Mickey's Once Upon A Christmastime Parade took place at both 8:15 PM & 10:30 PM (setting off from Frontierland) and that Holiday Wishes - Celebrate the Spirit of the Season fireworks spectacular began at 9:30 PM. Although exact times have not been released as of yet for this year, I would expect that these times will not alter much, if at all.
I would therefore recommend that you plan to make an Advance Dining Reservation at the beginning of the party which means that you'll have plenty of time to enjoy your meal before watching the parade and fireworks.
My tip to you would be to watch the second parade as you will find that many guests with smaller children tend to leave the Magic Kingdom Park once the fireworks have finished. You should have no problem at all in finding a good spot to watch the parade from, just before it begins.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma