I hatched in October, and 2 weeks later it was 20 below for the high. I almost pulled them. But I watched carefully, and they were doing fine.
The mama bird would change her cluck, and they all ran under her, and seemed to get a good warm up, and a few minutes later were running all over the outside run.
I only let her hatch 4, because I wanted there to be plenty of space under the broody. Another long term poster here, strongly suggested soaking grain in water, and adding that to the dry chick starter. The grains do not freeze together in a solid clump, even chicks, and definitely mama can break them apart. It worked for them. At 20 below, water freezes solid pretty darn quickly. I work so took it out twice a day.
Mrs K