If you really can't put on your heating & you have a tent, make up your bed in the tent.
Mattress & all.
The smaller space will be easier to heat (body heat in the night) & the air in the tent will get warmer than the rest of the room in the night.
Kids tend to find it fun & if you have your mattress in a tent (or size dependant you can set up the tent on your actual bed frame) it's really comfortable.
Back when 4 poster beds were a thing, those thick curtains did the same thing as you'd be using the tent to do, but better - cut down on drafts, insulate from the colder air & keep in any heat generated better.
When my boiler broke one winter, before I had back up heating or electric blankets (we now have all the things), we slept in a tent set up on the bed for a week.
It made a massive difference.
Set a tray in there for kids to use as a table (you do need a lamp to read) & when home they spent most of their time in there as it was warmer than the rest of the house.
A few years back I saw knitted leggings in a shop & I bought half a dozen paris.
They are fairly thick, so it's like you have a jumper on your legs but thin enough to wear under dresses or trousers if they are not tight ones.
Hats really help you stay warm as you lose a lot of heat from your head.
2 pairs of socks (a thin pair under a thicker one) helps, as does gloves.
Blankets on chairs to sit on (or a duvet) as well as to go over you means you will get warmer than letting heat escape into the chair under you.
I have a mesh computer chair which is fab in the summer.
In the winter I put a blanket over it & I have a small heating pad for when it's very cold.
If you have laminate/wooden/tile sort of floors, put a small rug (even a bath mat would do) between your feet & the floor even if you have shoes on if you will be sat there for a while.
Cold houses are the reason people used to have those little mats at the base of their toilets & sinks.
Insulating you from surfaces that will steal your body heat fast.
They might be unfashionable, but they still work.