I was the head of our SAR team's cliff rescue team for a few years, and I also taught our SWAT team members basic rappelling. We bought all our ropes and gear from CMC Rescue. We use 1/2" kern-mantle such as the Static Pro Lifeline and steel rescue 8 plates for rescues involving more than 1 person on line (ie: litter tender plus patient). For single person on line or remote back-country stuff we used 7/16" ropes and aluminum rescue 8 plates. We pretty much followed NFPA guidelines when it came to technical gear.
Over the years we started replacing the 8 plates with the brake-bar rack for the majority of our rescues. I'm a die-hard 8 plate fan myself because I like simplicity and bomb-proof, especially when it's cold, dark, wet and muddy, and you're going on zero sleep (the majority of rescues ;-). But even I will admit that the bar rack has some great things going for it, such as the ability to add and decrease friction under load, as well as it doesn't twist your ropes up as bad on long drops like the 8 plates do.
As for anchors, we use 1" tubular webbing, typically a 3 wrap, 2 pull, although where practical, a tensionless hitch with the rappell line is often my personal favorite choice.
If I could give you one piece of advice before taking the scouts out, it would be always have back-ups. Always use a safety line in addition to your main line. Secondly, always have a rescuer suited up and ready for a pick-off at a moments notice. When someone gets hung up or freezes up mentally while over the side, that is not the time to be practicing pick-offs or getting your back-up system in place. I've seen training/ play sessions go to "real time rescues" on more than one occassion because of unforseen situations, especially where novice rappellers are involved.
Finally, always have a safety officer designated. That person's sole job is to check every knot, every system, every anchor, every harness, helmet, etc. before people go over the side. Your system should be set up so that if a whistle blows at any time, every single person there can let go of what they are holding and the person on-line will not drop. You accomplish this with brakes and safety lines (rescuesenders, Gibbs, prusik cords, etc.). Nobody should go over the side in anything less than a full body harness. This doesn't mean you have to buy a comercially made one. A seat harness and webbing chest loop will work just fine. but the person has to be able to stay in their harness while inverted.
Sorry if it sounds like I'm coming across as an alarmist. I'm all in favor of teaching scouts rappelling (I did it when I was a scout). But at the same time, rappelling is an exceptionally dangerous activity (far more so than firearms training), and emergencies often do happen. Scouts are at the perfect age to teach not only how fun rappelling can be, but also respect for dangerous activities and how to be safety-conscious in general. I'd highly suggest practicing setting up and taking down systems, including the rappelling itself, on flat terrain before doing it for real.