Gather children in the doorway of a room leading out into a long hallway, have the light on in the room (or ask them to imagine such a hallway).
Point out to the kids that you can't tell exactly where it becomes totally black in the hallway, it gradually darkens. Just as some activities we choose aren't totally bad. But the more we choose these kind of things to do the closer we are drawn to that line of good/evil. And in the dark we won't be able to tell when we have stepped over that line.
So always stay in the Light where there is no question about it.
