Personally I'd turn the radiator/s in their room/s off.
Don't forget that they have a purpose grown fur coat on them 24/7.
My bunnies live in a hutch (which is in a shed) outside and the only extra heat they have is a snugglesafe pad each at night. Oh, and extra bedding. That's it. They've always been snuggly warm with that so your bunnies will be even snugglier!
Don't forget though if you open the window in their room for fresh air purposes that they may be in a draft, so make sure it's only open for say an hour or so. Drafts may make them poorly.