My 4 year old son came back from school yesterday and had some skin rash on his face. It's winter here at the moment. He never had skin rash in cold weather before. The rash gone overnight, but when he went out again today he got rash again. Is it

HI**
I live in Canada, many children here, including my own would get a rash like your son, we call it a wind burn, it's not really caused by the wind. Its caused by the cold weather9 ) the temperature in the air)  
and their delicate skin has no protection from the cold, then coming inside wares its warm, the skin dries fast (the skin dries out to fast, and it unable to generate its own natural moisture fast enough to keep up with the environment, if untreated over a long period of time and cause infections and may scar)
When my children were small, I would use mineral oil around their checks, nose and chin, and rubbing it into the skin gently before and after school, try this for a couple weeks, if it persists, see your doctor, and he/ she may recommend Hydrocortisone.
Having sensitive skin as well as my children, I know allot about rashes. Mineral Oil is harmless unless some one is allergic and this is rare, I also use Mineral oil on creaky doors, chairs and once a friends rodent wheel, so it wouldn’t hurt the little critter if it chewed on it. Mineral oil is also good for us adults who are prown to chapped lips, great for cracked and dry feet also, and works wonders for the cuticles at the base of our nail, both finger and toe, and removing make up**

Hope this helps you and your son*

Blessings
MoonShadow