If you feel it is important to take off the coat, you can keep blankets in the car just for covering them to help trap in the body heat & keep warm. You can get cheap fleece blankets for $5 at a number of stores, & either fold them over double, or use two.
* disclaimer - I am not a mom, I don't use car seats, & I don't know if this would pose any additional safety risks, it just seems a possible solution to the issue.
T.
ETA: Great question MommyJen! Thanks especially to Katrina A. & Wee12adopt for the links to articles discussing & illustrating this issue.
My niece is 2yrs, & I have a nephew/niece on the way. I will be sharing this information with my sister - living in Chicago, they will need to deal with these weather considerations when travelling for years to come!
Even though I don't have kids of my own, I know plenty of friends with little ones. Unless they are going to be playing out in the snow, there isn't a need for a big puffy snow-suit.
It seems like the appropriate solution to the issue of safety when travelling is to have one or two non-bulky layers, and a warm coat for travelling to/from the car. Then, the coat comes off, child gets buckled in, & fleece blankets draped over them.
I know it can be hard to keep the back of a car as toasty as the front, where most of the heating vents are. For those extremely cold days, or for vehicles with no vents in the rear, do a quick search on Google or Amazon for Travel Car Heaters. They plug into the power supply in your car & provide a heat-fan source of warmth for your smallest passengers.
Here's what I found in a quick search on Amazon:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3...
Hope that helps!! T. =-)