During WWII a lot of lifepreservers for the Navy were made from cattail fluff - the stuff that dries out on the end of the stem. The fluff is very water repellent and makes a very warm vest or blanket filler, but is not as light as goose down which is also good in cold weather. If used over wool, even when wet, it is very light but not as flammable as the synthetics. Most synthetics contain and give off very deadly gases if burnt, also. Please remember that nearly all synthetic wools, polyesters, etc. will melt or burn extremely easily, especially near an open fire. If they are on your body in 200 degrees F. or more they can cause a severe body burn. I've seen it many times as a professional firefighter or even by the regular campfire with novices, as most survivors will be. Please consider these factors before recommending any synthetic fabrics, especially for children. Most inexperienced people are very careless around candles and campfires and pant legs, shirt-tails, etc. can ignite very quickly if synthetic.
Offered by Woodie.
I think your warning would apply to almost all types of fabric whether natural or synthetic being close to fire. Cotton would certainly burn. Remember most clothing will be wet from the day. Fires will not be often made. It will take much effort to make them by chopping wood to find a dry center chunk. Many times it will not be worth the trouble. However, your warning about fire in general as being dangerous is appropriate. As far as using synthetics or not, I think the advantage of water draining out quickly that synthetics and wool offer along with the rot resistance more than outweighs the dangers associated with fire. One should always be prepared in case of fire. Have some water or a wet rag around in case of hot fire spitting out on a person. Another way would be to have a screen covering around the fire to stop this sort of spitting. As best I can recall fire fighters use a type of synthetic material that resists fire. Possibly we could consider this for some of the personnel that would be close to fires, whether these be children or grown ups.
Offered by Mike.
Have you considered kapok? I don't know the Latin name for this silky fiber and it came from a ceiba tree, the fiber (kapok) cloth from the seeds of this tree. In the olden days, kapok was used for life jackets, but since it is too expensive in comparison with foam rubber it is no longer in use for life jackets. Kapok is non flammable, and waterproof and also very buoyant. When I was a kid we tried to roll this fiber with a piece of paper and tried to light it like a cigarette and to smoke it, but it didn't matter how we tried to burn it, the kapok would never burn, to some extend it became black and that was it. In Southeast Asia this kapok is still being used as a filling for mattresses and it is also used to fill sleeping bags, because of its properties - non-flammable and waterproof. I believe kapok is much better than synthetic or wool, if we still can get it. I am not too sure if one still can get it here in North America or Europe.
Offered by Tian.