Other than insulating and keeping them in the warmest room, I'd also recommend the heat packs. All of which have been recommended, obviously.
My thought on the heat packs, however, was cupping them if need be and perhaps putting them in an insulated box or something to try and keep them warm -- sort of like when they're being shipped.
The other day I saw at Home Depot that they were selling small heat packs for hands and stuff for 99¢ each and they're supposed to last up to 10 hours. Could be an option.
Just thought I'd mention it.