I could not imagine a situation that would call for complete sedation. The level of risk, not least of which is respiratory sedation without a way to moniter and correct, would seem to great under any circumstance. However something call conscious sedation or procedual sedation might be appropriate for any child deemed unusually anxious about the event.
Perhaps evaluation by a trained child psychologist or at least a dive instructor that frequently works with teens?
Give the kids a Xanax and hope for the best?