I was in a similar situation recently, and what I discovered was that the chicks can get warm under momma, but they can suffer when running around. The young baby chicks will not ever want to leave the nest (and momma's warmth) to eat because it is too cold outside for them, and they may starve to death.
It becomes a more stressful situation for everyone because of the extreme temps. Momma has to eat a lot more to keep herself warm too. You can insulate with haybales etc. or give them a small nest they can sleep in to retain heat.
I had 3 little ones who grew too big to fit 100% under their mom, and what happened was one morning they ALL had purplish frostbitten heads because their heads werent' covered. They were blind because their eyelids froze shut. Well, obviously I felt horrible. I had to bring the 3 little ones inside my house with the heat on every night, in a cat box with straw, so they could recover from the frostbite. They are all fine now, no blindness or anything, but mother nature is right to hatch most chicks out in the spring and summer (not fall and winter). I would take extra precautions for sure.