Hello. I was a DVC owner for a long time and finally sold my points last year in favor of a more traditional timeshare. Anyway, before I got rid of DVC I did a lot of research into trading into DVC through RCI. Long story short, it is very easy (just short of guaranteed) to get any week you want through RCI for DVC, so long as you are flexible on the resort and you put in a trade request at least 12 months ahead of time for the most popular weeks.
Sometime last year, RCI assigned all trades a point value. Last minute trades can be had for as few as 4 points while the most valued week at the hardest to get locations will take 61 points. I believe 61 is the highest value assigned to any RCI trade. Any size DVC unit in Orlando from studio to 2 bedrooms takes between (25-29 points). ALWAYS! Of course 2 bedrooms get snatched up within a few days and the studios linger in RCI for up to a few weeks, but the units go fairly quickly.
DVC has not yet released December weeks into RCI. It would be best for your parents to set-up an on-going request right away as the December DVC weeks are expected sometime this month or next.
Yes, early December is popular for DVC members, but so is Christmas and 4th of July and Easter and those weeks are always available through RCI. The thing to remember with RCI is that the best weeks go to people who have an on-going trade requests. The weeks that appear on-line are left overs. Not many prime weeks are going to seen on-line because those weeks have already gone to people in line waiting for them.
I hope this helps but I would say your chances of landing an early week in December at a DVC resort are pretty good. You just need to get on a wait list.
Since this is the DVC board, I do want to note that so far, no one has reported being able to trade into BLT or GC though RCI.