Now plug it into a monitor and show the bounding box around each face. They'll love this!
Show them the code. GIve them a really high level overview : "Here we tell it to get a picture from the camera. Next we look for faces. If we find one we tell it send power to the LED to light it up."
Now, another interactive moment:
Plug in a USB keyboard and Ask a student to change the color of the bounding box. Of course they won't know anything about the code. Tell them to find the word "green" (or whatever) and change it to another color name. Run it again.
Little low-risk changes like that is how most of us learned to code. Maybe it will spark interest.