I usually focus on 3 main lessons...
1) Relax and draw from your shoulder. I want to show them how to draw a clean simple bold line not chicken scratch. I think confidence and control need attention. Often, the kids are still learning basic coordination skills with items like scissors or crayons. This lets them be a little more bold and less cautious.
2) Draw lightly first, darken up later. I want them to consider composition and variations before settling on their first thought. Often I scribble all over the page lightly to show how I am just trying to figure out what to draw. I ask them to hold the pencil at the very end as they draw to ensure they can't draw to dark. When I darken up the key lines the kids see how the under scribbles disappear and the bold lines stand out.
3) No Erasers! This is also about confidence. Often the kids seem to have, in their head, an idea of the "perfect" line they want to draw. This can spiral into a cycle of drawing then erasing over and over. I want them to understand that mistakes are a part of the process. It is easier to just ask them not to use erasers than trying to limit their use.
Above all, the message is to have fun. I try to show them good examples but leave them to determine how much they want to try. My only "demand" is that they try sketching something, not just asking questions. I may go in with Pirates but if all the suggestions are to draw a pumpkins, then we draw pumpkins! But, I may add an eye patch and a wicked sneer!
Keep Sketching!!!!