My son is having similar issues with his that are now about 12 weeks old. They have been moved into garage connected to house where they are dry and shielded from the wind. They did fine with mother but when weaned she quite covering them. They are showing signs of stress when temperature drops down to around freezing. If temperatures drop further then I will have him setup a heat lamp.Oh, ok I am struggling a little with some of my biddies. They are oriental so not known for their wintering abilities. And it does get cold here.
American Games are a lot tougher. I have raised chicks even during the harshest winters although that was done with birds in manger part of barn surrounded by occupied pens with livestock (cattle) that produced heat. Broods were actually large since did not have to tangle with cocci and rain that can be a problem in spring.