Hmmmm, there's a question! Probably, straw, as they can 'nest' in it if they want to (although in my experience they prefer not to!). Maybe the best solution (if slightly messy!) would be to line the hutch with vetbed, with straw on top? Maybe even a snugglesafe or 2? Lots of lovely insulation. Are they in a shed? If so you could get a heater.
Welcome to Rabbits United anyway! :wave: