Excuse me if I misunderstand, but aren't we talking about an instructor asking someone to teach
one class for a special occassion vs. giving a regular time slot to a low ranking student?
Those seem like very different situations. As a non-kyu holding student I would feel a little odd about taking a set class from someone one belt above me when there are higher ranks in the dojo....
But if I knew it was just one class, a special occassion, I would probably have a lot of fun with it- especially if it were a nine year old! That sounds like an incredible way to prepare for testing- I think it is a wonderful idea.
But giving that nine- year old one of your regular scheduled classes..wouldn't that be a bit awkward for the other students?
Though, if I were learning a whole lot from said class- I hope I could overlook the rank difference! I guess if they were just uber awesome I could overlook it- tough to say when you consider ego, dojo norms and ettiquette, and Sensei's goals/reasons. Though, if I were perfect, ego and Sensei's reasons would not be an issue at all.
This is a challenging scenario for me to think about, personally. Though the beer makes a lot of sense!