You will probably find an increase in crowds starting the 22 or 23rd. On xmas eve and xmas day it will be VERY crowded. Parks usually fill to capacity by 11am. Plan now for those days and make a PS if you wish to eat at any of the sit down restaurants at the 90 day mark. If you wait until you arrive your choices will be extremely limited (fast food only).
For most of your stay, crowds will not be that bad. I would suggest that every member of your party make a top 5 list of must do attractions or shows. Then concentrate on them. Anything else is just cherry on the sundae.
There will be lots of extra holiday activities. Take the time to visit the resorts and see the various decorations. GF is one of our faves. We enjoy the caroling cms and the xmas music provided by the band and pianist. You can sip on cocoa or munch on gingerbread and roasted chestnuts.
Have a magical time.