We had a product called 'Snowfire' recommend to us Lord Flynn. Comes as a small stick like lipstick, which we found very difficult to apply so went back to Vaseline. The object is simply to stop cooling by evaporation, because chickens lose half their water through their very delicate skin. Our Leghorn bantams come in when it gets to -4C forecast, because at that temperature even the vaseline won't help. Frostbit is indicative of a low core body temperature and I've been told that if cockerels get it they become sterile -don't think hens are affected.
One thing worth adding is that in our recent experience a low core temperature results in the death of the gut flora. They then can't get enough nutrient out of their feed to stay warm and we had to use Avipro Avian over a two week period to get his digestion to improve. So I think if in any doubt get them in, because once they get too cold recovery will be difficult.