When I was an instructor, one thing I found very helpful to kids (which is not something that was even mentioned in instructor training) is to tell them to focus on their weight when they turn. I didn't mention edging much because I think that's pushing it too far for beginners. First I would have them just try to turn, see if they can figure it out on their own, which would usually be by 'steering' their feet or whatever. After they tried that once or twice I'd tell them to not focus on steering so much and rather focus on their weight. Tell them that if they want to turn left, put 90% of their weight on their right foot, and to turn right, put 90% of their weight on their left foot (yeah, 90% is an exaggeration, but it will make them really do it, and telling them that definitely works; almost all of the kids I taught said that turning was much easier after I told them that).
Another thing beginners tend to do is try to steer with their upper body. They'll be twisting at the waist and moving their arms all over. Make sure they keep a still upper body.
Other stuff is pretty basic, you'll probably be told in instructor training what to tell the kids, but basic stuff like balanced athletic position, weight forward, arms out front, feet shoulder width apart, etc.
I would recommend against teaching snow plow as the primary way to stop. Teach them snow plow as an emergency way to stop, but teach them to turn across the hill (perpendicular to the fall line) or even slightly uphill to come to a stop, and try to get them to keep their feet parallel. There is disagreement on instructors if teaching kids to start out by snowplowing all the time is good or bad, but I take the position (as do many instructors) that it's best to start them parallel right away so they don't get into a bad habit, and so that they get the right balanched feeling right away.
When the lesson is over, I also liked to give them some advice on what to do going forward. What things to keep working on, etc. One thing I would generally tell them is to not start using poles right away. I would tell them that if they used poles right away they would rely on them too much as a crutch, whereas if they become a good skier before they start using poles, then the poles will just be a tool, not a crutch. Also it's a good idea to tell them to go on terrain they are very comfortable on before trying something new, and to try familiar stuff on more challenging terrain. For example, if they can do blues, but haven't got carving down yet, they should go back to greens when practicing carving. If they want to try a black for the first time, they should not try to practice carving or hockey stops or whatever at the same time, they should use techniques they are comfortable with when trying new terrain that is challenging to them.