If they are under age 7 or 8 and aren't part Eskimo, then they probably shouldn't be out in sub-zero temps. Nothing is going to help. If temps are a little warmer or child is a little older then dress 'em like a modern adult starting with good quality undergarments/long underwear, multiple thin layers of high warmth and wicking clothing on top and bottom, waterproof ski pants and jacket, good quality gloves/mittens with possibly a thin pair of liner gloves, neck gaiter and/or face mask, warm hat and possibly a hood. One good pair of modern socks and possibly a second thin pair underneath primary sock. Etc, etc, etc, all the usual modern stuff. A good ski shop can advise you on this, maybe start at a ski swap and buy good quality used clothing for kids. When you get experienced or kids get older, you can find good buys on stuff from non ski-specific shops or on internet.
I'm sure there are lots of old threads on this, here's one from just recently: http://forums.epicski.com/showthread.php?t=41387