Sometime early in the coming year I'll be introducing 6th graders to a way of organizing polyhedra, one inside the other, plus showing how their volumes might be related.
I learned this way of doing it from Bucky Fuller, the geodesic dome guy. I've covered the same or similar content with younger kids with good results, so I anticipate no problems. Various hands-on activities go with the projected slides 'n videos.
For those of you looking for a good on-ramp to polyhedra, I'll provide this link to a succinct synopsis of the content (don't worry, no popups, nothing for sale):