Keep one room warm when the heat is off

At the bare minimum, pick one room, gather blankets and have a safe way to add a bit of heat. That's how I'd start.

Heating a whole house without power is nearly impossible. Heating one small room is very doable. Pick a room, seal it, and put everyone and every heat source in it.

Bodies, a small safe heater and good insulation can keep a room 10–15 °C warmer than the rest of the house.

YOU'LL NEED

A small interior room, ideally with few windows
Blankets, tarps or moving blankets
Painter's tape or thumbtacks
An indoor-rated heater (propane or a power station isn't enough for resistance heat)
A battery carbon monoxide alarm

Instructions

Pick the right room. Choose a small room on the main or lowest floor, on the south side if you can, with one door and few windows.
Close it off. Hang blankets over doorways and the openings of halls. Roll towels at the bottom of doors.
Cover the windows. Tape bubble wrap or a clear plastic window kit to the glass, then close heavy curtains or hang a blanket at night.
Insulate the floor. Lay rugs, foam pads or sleeping mats. Cold floors steal a lot of heat.
Add safe heat. Use only heaters rated for indoor use, follow the manual, keep 1 m of clear space around them, and crack a window a little for fresh air.
Set up the CO alarm. Put a battery carbon monoxide alarm at sleeping height in the room. Test it before you light anything.
Layer people, not just the room. Wool socks, hats, and sleeping bags inside the warm room. Share beds and put kids in the middle.