Here is what we do at camp when it's that cold... (they are only snapshots BTW...)
In the morning, we pile up snow in a mound thats around 7-8ft high and let it settle. Then in the afternoon, we hollow it out. It's called a Quinzee (sort of a non polar version of an Igloo).
And then at night, 4 of the kids sleep in the Quinzee. It's -27C outside, but a quinzee insulates so well that it's never below -5C inside. We had to poke vent holes in the ceiling on this one as the temp got up to +5C.
To give you an idea of the size of this shelter it slept 4 very comfortably, and had enough room for 12 of us to sit in it with room to spare.
And of course, the next morning when the Quinzee is no longer needed the kids have a blast destroying it. The snow dome by now has turned to ice and is so solid that they can stand on it. They have to jump with both feet to break it.
And thats how I spent part of my weekend with a dozen kids
Pat