We've taken the kids 5 or 6 times over the past couple of years. My wife is gung-ho on the Disney stuff. I HATE lines, I'm one of those people that enjoys a larger than normal personal bubble of space around me.
So to address your question, we generally try to find the off weeks to go there. For example, we are headed there the week before Thanksgiving. Any time kids in school have days off, people flock to Disney.
If you went 1st week in Dec, I would suspect that this would be a much slower week seeing that the week before was a prime week with Turkey Day. I think April could be hit or miss...most kids would still be in school, so it shouldn't be all that bad...better than spring break in March.
Hope this helps!
Boat: Med Blue 2012 X2
Old MC: 1998 Maristar 225 VRS
Rig: 2009 Chevy Tahoe LTZ
He who dies with the most toys wins...I'm losing right now!!!