The first rule is that when working with young people in climbing, it is vital that the instructor always be serious about the systems components. While joking around a bit with the students can build comraderie, that cannot extended to a lack of seriousness about an alert systematic approach to climbing. To joke around in this area of instruction is to invite further joking or carelessness by the scout, as scouts will always joke more, and be less careful, than the adults they see. As a scouter, you must avoid the temptation to be as "relaxed" as you would be on a personal climbing trip as this sends the wrong signals to those with less experience and skill. They need to pay careful attention, the way a new driver needs to pay more careful attention to the road. This is not to say that you should become rules-bound or overly stern; consider it similar to gun safety; when you’re holding a firearm, loaded or not, fooling is not appropriate.
Considerable research has shown that in the medium- to long-term, we only remember well the things in which we have been over-instructed. More than regular scout activities, boys must be over-drilled in rope & rock safety. For this reason, it's important to re-teach things several times in different environments, so that the boys both learn things well and see the application in different settings/locations. In the skills progression list, you will see that the instruction "spirals" Things are introduced at one session, taught again and in more depth at the next, taught again and reviewed at a third. This written spiral is in fact a minimum; in practice, the troop program does even more review and re-teaching than is listed. Don't fall into the trap of believing that because you've taught it, they've learned it.
Think carefully about the venue in which you choose to teach. Climbing is a fun activity to watch, so doing beginning-level instruction at a gym or popular area is a sure way to make it hard for the kids to pay close attention to you. For this reason, we do basic instruction on flat land, away from active climbing.
Small groups are not just best, they're necessary. One competent experienced person for every 2-3 beginners is a good ratio; you can use your experienced older boys in this capacity along with other adults. This gives you one experienced hand to provide hands-on coaching for each climbing team. Troop 8 rarely has more than 8 or so relatively inexperienced kids out at once, in fact 8 would be high. When we do, we switch to a more "institutional" approach.
Not all experienced climbers (kids or adults) make good group instructors; speaking ability, organizational skill, comfort, personality and other factors play a large role. In addition to an experienced coach for each climbing team, you should think in terms of one competent adult with good instructional skills for every 10-12 boys (max). This person does the lecture/demo and coordination, and sets the tone. If you're going to have a lot of people out, though, make sure you have enough equipment so that everyone can practice immediately. It's better to break into smaller groups at different session times than have only one or two ropes to share among 12 people.