+1 on Les' recommendations.

I would consider interspersing lecture, demonstration (where practical) and the boys doing their work or orating back (as that fits the outline) while keeping each lecture shorter; perhaps 20-30 minutes max for each where possible and coupled with a 5-15 minute demonstration where appropriate and practical.

I did some similar volunteering for both my DS's and DD's troops several years back and I swear (and this is just my perception) that young people's attention spans gets a bit shorter every year.

You'll be able to gauge after the first one or two talks to know if you need to go longer or shorter.

As I have done just a little bit of instruction at the university level over the years I learned way back then that there are several different learning types: auditory, visual, self-reading, experiential, etc; and most people learn best with some combination of them. Not sure if you're working with 5 or 50 kids but you'll probably be able to pick up on their preferred style(s) as you go along.

I used to be mainly an auditory style learner, so I wouldn't make a lot of eye contact with the teachers. I would tend to look down at my desk with an ear towards the front of the classroom. Many times I was told to "Pay Attention!" (no "Please" or "Thank You" back in those days )and early on I was confused because that is what I was doing, just more with my ears than my eyes. Looking back, I think averting my eyes was my way of cutting down on the visual stimuli and focusing more on the auditory. Over the years I learned become more visual.

I believe the following is attributed Benjamin Franklin: “Tell me and I forget, teach me and I may remember, involve me and I learn.”

Good Luck and have fun with it.