What I did to keep the kids warm were 2 things: either run the furnace, use a heating blanket, or use multiple warm blankets. If I used the blankets, I also covered their head with a warm hat, the kind without a brim that might unfold down over the face. Whatever you do, and you seem to already understand this, listen to the wife here. If you end up getting a space heater against her wishes, go ahead right now and prepare a space outside for your future bedroom.
Good luck.