Rock climbing is one of those scary skill sports where a youth’s athletic ability and personality make a big difference in the initial sessions. You should be prepared for a wider range of instructional needs than is normal in a scout environment. A few boys may be moderately acrophobic; some strong kids with limited balance skills will be frustrated, while a few lightweight, wiry kids will fearlessly blaze up nasty climbs.
As an instructor, you need to be prepared for this range, and work out in your own mind and with your fellow youth and adult leaders how you are going to manage this. Who is going to handle "coaching" the scared, stuck climber(s)? Handle the ones who have trouble even getting started? Who is going to manage the other kids, or the accomplished kids? Everyone will follow your lead in terms of being patient with and encouraging each other, so be thoughtful in advance of how you are going to handle difficulties.
Sometimes it’s difficult to decide when to push a kid, and when to lower him or suggest that he "bail." A good rule of thumb is the notion of "training ‘til failure." If a kid is stuck on a move through 3 good attempts with no success, it does no good for him to continue to thrash; indeed, it works against developing good technique and mental fitness. Suggest (but don’t force) that he quit and rest for awhile and then come back to the climb. This helps the kid to save face ("I could have kept going, but Mr. G. suggested I take a break before I tackle it again, and that was a good idea."). An alternative, depending on circumstances, is to have him take a break on a nearby ledge while you coach the needed technique, then let him try again. This allows you to judge his emotional state, and suggest a longer break or encourage a final effort as appropriate.