This skit works best if you have the participants up in front of the group in a "normal" setting. The fewer people who are "in on the gag" the better.
Need 3 participants - preferably leaders/counselors rather than campers (the effect is much better)
Participant 1 should have a decent sized piece of a greenish/yellow fruit roll-up laying on their skin - "the scab".
Participant 2 should be sitting next to Participant 1 - repeatedly pointing to scab and whispering/poking at it to Participant 1.
Participant #2 should increase the frequency and/or urgency of their interest/poking/eyeing of the "scab" to the great distress and aggravation of Participant 1 - to the point of having to slap P-2's hands back several times and being a great distration to the overall event.
Participant 3 should be doing normal activities in front of the group - explaining the day's events or whatever purpose the group has been called together to hear. Because of the distraction of P-1 & P-2, P-3 explains to everyone that P-1 fell down and skinned up their leg/arm/face (wherever the "scab" is located) - make the story elaborate and embarassing to add drama.
Just as P-3 finishes explaining how the scab came to be....P-2 should reach in, pluck off the "scab" and eat it.....shock, terror, joy, gasps will ensue.