Why does aileron response diminish as speed becomes slower, from slow-flight up to the stall? (In a light trainer)
This is real easy, no explanation required:
Tell your student to stick their hand out the window of the car driving 25mph in the city and rotate it like an aileron.
Then have them do it on the freeway doing 60mph.
Feel the difference?  Did smaller movements produce larger effects at higher speeds?
Practical examples are typically more effective at driving a point home than a lot of words.