I don't think you're really going to find any period in December when there are low crowds in the parks. It's a popular time in DLR, with all of the holiday decorations and entertainment, and other offerings. You might have better luck in November, but again, it's hard to say if this year is a good predictor of next year. However, if you go to early in November, the holiday season won't have started so there might be attractions closed, limited entertainment, etc.
All that being said, I agree with gottalovepluto, go whenever it fits best with your schedule. Expect and prepare for crowds and if you happen to get lucky and be there at a time with less crowds, take it as pixie dust!