Give extra hay for eating and if using straw for bedding, throw a little extra of that in as well. If you have a special pet that you like to bring in the house to play with, time to stop that. Taking them in and out is not a smart thing. They can easily get overheated while indoors and then getting put back out in the cold is an unkind and risky thing to do. Would you wear your winter coat indoors or go outside without one?
The good thing is that the bunnies aren't molting. They are holding onto every piece of fur they can and are going to need it for the months ahead!