I stated at the front of my first post on this that I was at CS1. Should have spelled that out as it is for Child Specialist. There isn't a lot of "maximum velocity" in that course.
This forum is on its second season debating experts flexing their ankles and many of the senior ski pros, to include coaches and instructors from CSIA.
I wanted the ski to go flat so they could turn it. We weren't carving. We were learning to steer a turn - press your toes to the left to go left. Ski has to be flat for that to work.
So back to the original question. How do you teach femur rotation?
If you are coaching/instructing/teaching children up to the age of 4 maybe 5, there center of gravity generally is quit high. 'big head little bodies' once they start sliding they are BAM back seat drivers. they will use rotation to change direction do to lack of centered balance, coordination and muscle development.
IMO let them go. Have fun and keep it safe.
When you think they are ready for some further skill development, as per this thread. Play games to create a desired movement.
Games such as, squish the bug (flat ski) or spreading jam on toast with your skis.
Anything that will get them to make the first move, to turn, happen with the foot, up the leg and that the ball joint in the femur is turning in the hip socket .
As the kids get old it becomes an easier concept to understand and accomplish. Separation/steering/skill of pivoting.