Position Diagrams
If you do nothing else, look at the roster chart and download it or use one your league provides. To understand why read “Talking less and Showing More” as well as “Never Leave a Kid Confused” in my idea section
The Diagrams aren’t necessarily only about Soccer, and in fact, at some level of understanding of the game the idea of positions isn’t really what they need to learn. The key is to remove doubt and uncertainty from kids new to a game: letting them know they’ll be playing and where they will be playing which heads off many issues and is worth losing some degree of flexibility(which you still have anyway).