The adult rabbits will be fine well below freezing. Just add more hay for insulation for the mothers to use as they need. If it's inside a building you don't have to worry about wind or rain which is more problematic. That's what can affect their health or kill them. Not the cold. For the babies if the moms are pulling more fur, they are making thicker blankets which is great! As long as the kits stay under the blankets they are fine. I do all my breeding across fall, winter and spring because summers are so hot. When I stick my hand over the fur blanket I feel tremendous heat radiating off the kits underneath. They are all piled up and toasty and warm. The thing you do have to watch out for is any babies that are still latched to a teat when the mother hops out. If they are too young to get back into the nest box they will die. I was having problems with finding young kits outside the nest box last year, and I took the advice here and I made a little set of steps with half bricks at the end that they were able to hop up to get back in, and that solved that problem!