It really depends when during Christmas you go.
The most crowded days are between Christmas Day and New Years Day.
We've been going almost every year on Christmas Eve, and it has never really been a problem. Most things are 25 minutes or less, except for Holiday only attractions.
I would say that 2 days should be just fine...one day for each park, or throw in a third to Re-ride the things you liked.
Tips:
Luminaria should be back this year in DCA. The fun thing about this, is that you can draw and color Christmas greetings that may be featured in this show. (Most others that aren't in the show, will be shown afterwards, so you can wait for your kids ones to show up.) If you don't want to take time to draw in the park, they usually have a link on their webiste to get the basic form, and then you can do it at home or on the plane or car, and then turn it in when you get there. Just remember to see Luminaria on the day that you turn it in.
Other attractions aren't horrid, but remember to do the Holiday Specific attractions early or late in your day. (you can also do it during a parade or show if you like).
Some folks say that they like to do Small World Holiday first in the day, and though the inside is the same any part of the day, the outside lights aren't on.
My personal preference for this ride, is to watch believe from the steps of the small world promenade, watch the snow, and then get to be first in line at Small World when it reopens. Usually it works for us, (even though I prefer to see Believe in Holiday Magic from Main Street - It just seems cooler).
Your kids may be to young for Nightmare before Christmas (Haunted Mansion Holiday), but if they want to do it, maybe you should do that first thing in the morning, so they can come out and see daylight. In my humble opinion, HMH is much more frightening than the normal HM.
Make sure that you let them decorate and eat their own cookies...very spiffy.
Anywho, have a great time
I wish you the best
~Tina