OM has pretty much said it all. Unless you get a canvas tent with a wood stove like some winter campers use in very cold weather (- 30F) or so, you want to dress warm, have a good bag and pad under it and eat a lot. For a little one, I would think keeping him warmly dressed and well fed would be more important than anything else. Make sure he has good footwear, a fleece or wool beanie, and warm gloves or mittens. He's not going to generating a lot of heat hiking like an adult would, so make sure he has a warm jacket and probably insulated pants. Fleece pants work well. I have a pair myself. If he winds up looking like the Michelin Man, he's probably good. Chocolate and a hot drink before bed or if he likes them, part of an energy bar like a Clif Bar or Powerbar. You'd be surprised how well that works in cold weather.