If it were me, I would...
Take a plastic tote, fill it with dirt, (Could just use somewhere with mulch as well) take some items like coins or other trinkets, and put them down in the dirt.
You might look up online how it is that a metal detector actually works, and then maybe hand out a paper to each scout with the basics of what you are demonstrating.
Let them take turns swinging the detector, and see if they can get the detector to sound off on the item (of course they all will, as you only buried it a few inches deep.)
Let a couple of them get the thrill of hearing it beep, and finding "treasure"
Likewise, have some "awards" printed out that they all receive for their daily lesson in becoming "certified treasure hunters"
When you get a few of them hooked on the hobby, give us a holler and we'll be happy to work out a bulk discount on some fancy treasure finding metal detectors for your scout troop.
Best of luck, snap some pics and share...we love pics!