When I was young I had someone hand me and a couple of friends scythes and tell us "go cut that big patch of grass and weeds". It was fun, We stayed well away from each other, but within easy talking distance. After a while we figured out how to spare our backs, and we got the area mowed with no injuries.
Things to keep in mind when you are giving your 5 minute safety demo:
1. Keep the mowers well away from each other! This is where really bad injuries will happen. I would be afraid to have anyone swinging a scythe near my precious ankles, calves and achilles tendons, especially if they are tired or inexperienced and foolishly confident. A couple of years ago I was mowing my hillside and accidently decapitated/ disemboweled a couple of baby cottontail
rabbits in a clump of heavy grass, (traumatizing my kids, not doing a lot of good to any of the young rabbits that survived and ran off), so injuries to anything you are unaware is in the grass is possible and likely.
2. Show them how really sharp the blade is. If that doesn't teach respect, nothing will except, maybe, blood.
3. Blade abuse will happen. Warn them what not to do of course, but expect blade tips in the dirt, some too heavy brush to be chopped, and maybe (although I really hope not) a rock to be hit. Look at your blades, your mowing area and your people and decide if you are willing to deal with it.