What you are seeing with the chicks is normal. Broodies can raise them even if the temperatures drop below freezing. Obviously it's better if the temperatures are higher, but even when it is pretty cold the chicks go out to eat, drink, scratch, and play, then go back under her when they need to warm up.
Many of us brood outside, a few of us even when it is below freezing. As long as the chicks have a place warm enough they are really good about self-regulating their own heat requirements.
