Hi Maz - I walked in Jan/Feb 2013 and was an awesome experience. I'd certainly recommend the CF over the CN just because of the Albergue options. On CF there were plenty available in the winter months, never was a real challenge getting a bed for the night. CN will be tougher and likely you'll need to mix in plenty of Pensions or Casa Rurals which will run up the price somewhat. The first couple of days were a challenge in the winter with snow up to my waist towards the top of the Pyrenees on the Valcarlos Route. Day 2 was really bad for ice, so much so that I'm taking a pair of Icetrekkers - Diamond Grip to help keep me upright. After the snow and sub-zero weather it was great walking after that with snow reappearing around O'Cebriero and some pretty heavy bouts of rainfall. Quality rain gear saved my bacon but my favorite cold-weather piece of gear was my merino wool buff. Made everything really manageable.
One thought about walking with your 12 year old son is that the days are cold and long. For adults it seemed pretty manageable however that will be pretty rough on a young person I think. I hope with 8 weeks you can accomplish this together!!
Buen Camino,
Jordon