The time they will be there the crowds are still on the low side. Most of the Christmas Crowds do not get there until 22-25 or so. We go this time (before 12/21 almost every trip and there is no large crowds there yet. Crowd wise they will be fine.You must plan carefully, the nearer to Christmas you get the crazier it gets.
Pack for all weathers - we were in shorts and T-shirts during the day, take a sweater for evenings. Dont forget your swimwear
You have to see the MK parades and the fireworks, and the evening snow on Main Street
Register on WDWMAGIC. This sidebar will go away, and you'll see fewer ads.