I do know of a situation where some kids were buckled in and died of hypothermia (didn't have their coats on, high school kids driving desolate winter roads in Wyoming). The parents were told that the kids would have lived if they'd had their coats on to keep them warm 'til their wreck was noticed. They didn't die from their injuries, but from hypothermia (coupled with broken limbs or etc. preventing them from doing anything to GET warm). They were friends of my cousins.
.... I think layers with fleece and bringing the winter coats along is a good option - start the car to warm it up before you go and bring the winter coats with you if you're going to an outdoor activity (meeting friends at the sled hill or whatever). Bringing a warm blanket to toss over kids once they're buckled but doors are still open/vehicle is cold, is also a good idea (I had a drafty old car that I used to drive with a blanket over my lap and legs, when I was in high school and college!).
Thanks for asking the question, and for the blog link. I was a little skeptical 'til I read the link - and showed my dh and thought he'd be skeptical but he's all for changing how we do things. I'm not worried about hypothermia or exposure if we get in a wreck while we're running errands 'round town!
ETA: Where I grew up, high in the mountains in MT, I remember regularly (nothing anyone every commented about, it was just the normal winter thing) temperatures of 40 below (F and C are the same at that temp) - sometimes lasting for a couple weeks without ever cracking above 0 and they never calculated the wind temperature on the forecasts they'd just say "plus windchill.") So, wanted to clarify that while OP is talking about cold weather and probably has longer periods of cold weather than those of us to the south, there are parts of the Lower 40 that still can become very cold. That said, it never got that cold when I was by Lake Michigan - lake effect, I don't know? ;) I think my solution is one I'd use, even at those very cold temperatures - layers, fleece on top, warm blanket to bundle 'round them while whisking into vehicle as quickly as I can, and preheat the car before we go anywhere so it has a chance to warm up. The blankets are also nice because when it is that cold, no matter how good the car's heater is, there is a cold, cold space near the windows.
And, the cold is also a major reason we went with a bucket system for our infant seats. We would use one of the fleece covers on the bucket seat, load her in the house, tuck 1-2 blankets around her, fleece cover over all - they never got cold - by the time the cold would have penetrated, the heater was warm enough that they were fine.
Most of the time when I'm traveling, I'm in town so if there were a wreck, hypothermia wouldn't be a concern - there'd be people right there to help immediately, so the fleece and blankets would be fine. When we do longer road trips on winter roads (which is frequently) we always have winter coats and blankets right by the kids so we can cover them quickly and we do so if the roads get dicey. We travel a lot on isolated roads ....
I do think it's important to also remember, snow boots and hats are still going to be keeping a lot of heat retained on kiddos. If they're wearing a good warm hat and their boots (and maybe mittens) that will help a lot with the cold temperatures as the car is warming up more while you're driving. We lose a TON of heat through our heads, the hats help!
Edited by elanorh - 12/9/10 at 10:33pm