The people that say they never have the heating on overnight are utterly missing the point here. When you have the heating on during the day or evening, or you heat the house in some way eg by having a fire, or running hot water in the pipes, or having a a space heater you are introducing heat. The house will benefit from that residual heat, throughout, for days afterward, even if just a little. If you then heat the house again the next day, it will boost it again, and on and on it goes. The house will never be allowed to become cold in the way a house with no heat source becomes cold. Earlier in my life, with a small DC, I had no form of heating and no form of hot water in my home. No baths, no showers, no warm hand washing, no heating, no fires, no warm water running in the pipes or radiators, no fires. Just one small space heater that couldn’t be run with a baby or toddler DC in the room as it glowed bright orange light and would burn you if touched, and only enough money to put it on here and there for max 20 minutes if we were very, very cold. It reached 3 degrees indoors that winter. We had chilblains. The air was cold, the furniture was cold, the walls were freezing to the touch. You could see your breath, inside the house. The damp and cold went into the clothes and soft furnishings which never felt fully dry and always smelled fusty. The rooms went mouldy with black mould inside the plaster and white mould on and running through the carpets. The floorboards couldn’t dry out from cleaning and started to rot on the surface. One day, the pipes froze, and then burst, which sprayed ice cold water throughout the house and flooded. There was no money for home insurance. There was no money to go and stay in a hostel or to have the leak fixed or the damage put right. The house nearly rotted around us and took years, and thousands of pounds to fix, in the end. That’s what happens with no heating and no hot water. It’s not as simple as putting on an extra jumper, or even remotely the same as a house heated daily losing a little temperature overnight.
OP, I would sleep in the same bed as my DC to stay warm. We’d try to stay in the same room during the evening as we would sleep in so we could warm it up as much as possible. DC would wear the fleece onesies as we could never get cotton to feel dry, two fleece all in ones with socks under, then their coat on before bed, coat came off a second before jumping under the covers and we had a fleece blanket first again for damp reasons, then the duvet, then two more blankets on top, heavy ones. I didn’t have hot water bottles or hotties but I would definitely have tried those if I could.