Just a thought - but what about those liquid crystal hand warmers. The keep warm for quite a while and they would fit in a bag of recorders/whistles. Especially if the bag was well lagged. I used to have two of the things - they would work for about half hour each.

Body heat would be the easiest for one instrument but if you have several and you are only playing one at a time - it is an alternative.

Joy is learning the recorder and I was looking for info on range for "alto-soprano-descant".