Would this be warmer then an balaclava for winter camping trips?

That depends... Some balaclavas are warmer than others, from what I've experienced. But they pretty much are all designed to keep the face shielded from wind and varying degrees of cold, where as the ushanka is not meant for keeping the whole face warm. That said, this ushanka keeps the head and ears warm and cozy. The balaclava I have does not keep my ears and head as warm as my ushanka. Perhaps you could get an inexpensive, fitted balaclava AND a ushanka for a cold weather camping trip. A fitted balaclava would go under the ushanka if need be...

What size would fit a 13 yr old?

Measure your head with a tape measure, and use the size table

How do you put the pin in the hat?

I measured and found the center of the hat face and marked it with a small piece of masking tape. Then I got a sewing needle and made a small pin hole in the area of where I marked the center. I poked the hat pin needle into the pin hole, then carefully remove the needle and small piece of masking tape. I secure the hat pin and my Ushanka is all good.