I agree with you that the teacher should not rely on pictures of concert pianist sitting straight to motivate great posture but rather guide to student to explain WHY we would want to play things a certain way.
Ah.........no, I'm going to disagree with you on this one, because of the age of the student.
To an eight year old, an adult authority figure is only slightly less powerful than God. Most of that power is over emotion of course, not cognition, because of their developmental age.
YOU need to know why the rules work. (and in many cases, the rules are simply tradition and have no basis; but while this will drive a 14 year old nuts, it is meaningless to an 8 year old)
The child does not. It isn't helpful to an 8 year old to know why.
But worse, you're going to explain it using words. With an 8 year old, while you're talking learning stops.
(that is somewhat true at any age. But older students can handle more words before shutdown; younger ones cannot. For every student words begin to detract from learning after the point is made, and most of us talk long past that point without realizing it.)
On the other hand, 8 year olds are sponges with enormous capacity to learn by imitation. We lose that gradually as we age and it's almost unavailable to adult students.
If an 8 year old is inattentive, it's probably because your mouth is moving.
If an 8 year old is unmotivated, it's probably because you said, "don't do that" instead of "do it like this."
"Watch me." two words. "Show me." two words. "Do it like this." ah, four words, probably pushing the envelope, but short enough to maybe work. maybe. "Good job!" probably can't say that enough. But only when it's true. But it's always true for something, if you look.