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If you're worried about staying warm at night you should upgrade your sleeping bag. Also consider a liner and a cover, they will protect your niece expensive sleeping bag and add warmth.
I have a few different brands of long underwear. I think outdoor gear lab rated smartwool as the best you can get (also probably the most expensive), but they often only test the meme brands. I have a pair of 'expedition weight' from ll bean that I use only to sleep in, also damart are good. I bought warm terramar stuff when it was on sale but it's not as warm as I had wished. I have a Patagonia mid weight pair that is my favorite for hiking in, and very comfortable.
