H.G.
I don't have any suggestions on the warmth part, but I have used safety pins to hike up a costume that was too long for my daughter. I used big ones and threaded the fabric through at regular intervals to create a gathered sort of look at the bottom. You could also use a needle and thread and pull it in and out a few times vertically for the same effect. You'd need to do several of them around the bottom of the dress to make it even. Once the dress is shortened, you can wear warmer boots/socks.