If they live outside in the winter it is not a good idea to just bring them in when it is really cold. That can mess with their ability to cope with the cold and might make them start to moult early leaving them vulnerable to chills. It is better to permanently make them house rabbits, or just for the winter, or to invest in a good quality cover for their hutch. I covered all my hutches with thermawrap last year with a waterproof cover over the top. The waterbottles didn't even freeze when it got really cold. The rabbits spent all day in their runs (with access to their hutches) as they had adequete coats to cope with the cold, I only kept them in when it was too wet.