I voted BCV because it will probably be the easiest to get at 7 months (I'd call day by day). In addition, BCV has the best pool if that is important to your kids. Be forewarned, swimming can be an iffy proposition that week. Two years ago we were at WDW for Christmas week. It was absolutely too cold to swim on all but 2 days. We are talking high temps in the 40's and 50's, not 60's. It was an unusually cold week.
Tha being said, I highly recommend trying to get VWL. The Christmas atmosphere there is unmatched in any other
DVC resort. They are all nice, but VWL is incredible. The main pool is nice at VWL, but nothing like SAB. However, the guyser is really neat. The quiet pool by the villas is also nice - it bubbles. The hot tub by the quiet pool - the ultimate in relaxation. The boat to the MK is neat and convenient.
To be honest, we are splitting our early December trip between VWL (5 nights) and BCV (3 nights). Best of both worlds. However, if we could only do one it would definietly be VWL. We switched a VWL ressie to BCV for the three nights. Seems a lot of people are doing that to see the new BCV. Hopefully that will create some availability at VWL, which is pretty tight right now.
Good luck with your decision. As you know, you can't go wrong.
BTW - get the AP's. Then, if you don't do VWL this Dec, you can always stay at the VWL earlier in Dec next year. The week after Thanksgiving is the best.