if you don't show them how to do it cheaply, they won't listen.

Even having no direct experience as a scout master, this sounds 100% correct. Luckily, the lightest thing in your pack is always what you didn't bring, which is usually the cheapest, too. That's always the place to start with lightweight packing anyway.

Darn few of these kids will end up with a really good down sleeping bag, but I'd strongly urge the idea that, of all the places to put one's money, to reserve the lion's share of what you can afford to put toward the sleeping bag. Make a few suggestions about what a useable bag looks like when shopping for it, and provide some illustrations from real life: cite a few bags you located on the internet.