Ha ha, so I had everything set up and the wind ripped my tarp in half, 60mph gusts made short work of a tatonka 3x3, we didn't have snow on the ground then (it had all blown away off the frozen ground) but there was a lot more due over night, it was -7 and -17 with the wind and I binned it, I'll have another go soon
I've laid in it a fair bit but not done an overnighter yet, while in it, it's was toasty warm and that was in about -3c, I've a lightweight down blanket from costco and that was fine in there. I also noted that the kids could literally encapsulate themselves in the insulated base, it would come right up the sides and if they had a way of holding the top edge together they'd not need anything extra at all, I'm so wide that the sides come up far enough but that's it, slim people will get a lot of thermal protection with higher sides, even laying at a diagonal.
So, still looking forward to getting out, need to dig out an alternative tarp!