You have a nearly ideal group of children. Fifth through Eighth grade is about right. By the time they start high school, you're likely to encounter counter-productive attitudes based on what they've seen and heard in the media and (regretably) the classroom.
Of course you'll go through the steps involved in use of a Civil War-era firearm. Whenever possible, I offer the Enfield rifle I brought to a student and let them hold it. The look and feel of anything "real" like that makes an impression that no amount of lecture or media can produce. My best memory of one class demo was the look on an eighth-grade girl's face when she took that weapon in her hands. No amount of lecture or media could have made that impression.
You can't explain everything about it, and you'll lose the class if you try, so move on to *what* it does. You're sure to have both round .69 balls as well as minnies. Some students may already know about rifling inside the bore, but most have never thought about it. They'll immediately understand the difference between a ball that spins and one that doesn't, and recognize the improvement of a minnie shaped to spin through the barrel.
Now let them look over the samples you brought for this part of "show and tell". Then, while they're handling the musket and rifle balls, explain what soft lead like this does when it strikes something hard, like a bone.
Add a few other things about how companies organized and elected officers. Ask them to imagine young boys from their neighborhood assembling to go off to fight. What kind of ceremony would there be for them when they left for the army? Would the young ladies make a flag and present it to them?
At this point, with prior permission to use a DVD scene, I might show a scene from "Gettysburg" or "Gods and Generals" showing troops in line in the field --
-- sample --
We'll pray for you, and you let us know how this goes!