Jackie... there is pretty good educatonal research to change your order of operation a little... have the Cubs tell you first what they know about survival shelters, signaling, etc... and let them "buy in" to the idea you are presenting... then show your examples

assuming a trash can liner or two will be included...weather permitting...have them investigate how "cold" they feel with bare head, then compare... tie a "do rag" or "babuska" head covering with their bandana...cut a corner off a inexpensive can liner (for demonstration) for a breathing and vision hole (add a pair of inexpensive EMT shears until they get their knife permit)... to slip it over their head to conserve heat...nest to a lower can liner

signaling...three blasts on whistle the a 10 Mississippi wait.. my students came up with a pretty efficient passive signal ... I gave them a playing card.. the most effective was covered both sides with silver reflective mylar (potato chip bag)and bordered with 3M reflective tape on edges.. small hole in corner, and snap swivel and mono fishing line to attach to tree limb to move in the wind... set up a pair of adults at a distance huddled at the base of trees... one with the signal and one without...if you could do in daylight, and a flashlight at night I think it would be an memorable demo...

it normally took me a couple of years to fine tune a chem/physics lab... make sure you do your demonstration beforehand to work out the kinks... and an answer of "it doesn't work" is not necessarily wrong