Hi Priscilla!
How exciting!! First trip to Disney, during Christmas, and a present for the children?! You get the Mom of the Century Award :-)
As someone who's taken his family the last 2 years around this time, I'd say expect a steady crowd at all the Parks during your time there. The 17th-23rd is pretty manageable, Christmas Eve (the 24th) is a little busy, but Christmas Day and the day after Christmas is INSANELY busy! On those 2 days in particular, wear some soft-sole shoes and be ready for extra wait times, because it gets close to maxing out on those days.
However, for the most part, it's not too bad because it's Christmas time, and most everyone is in a pretty good mood, so it makes the extra wait a little more bearable...
Lunchtime and after definitely picks up in people-traffic, so I'd recommend getting to a Park early each day, and getting on the rides you enjoy the most early and first, as later in the day it's going to be tougher to get on.
Have a Fantasmic first trip and a Merry Christmas!