I'm teaching a nine-year-old how to play piano. He's very quick at learning, but he can't seem to pick up how to sit at the piano properly. No matter how much I prod him, he keeps slouching and dropping his wrists on the edge of the piano. How can I teach him to sit properly?
It may well be that the seat is at the wrong height for him. Raise it until he looks about right. Bear in mind that because humans all have different dimensions, there isn't a text-book posture. Each of us will find our own best position, although I agree that low wrists won't have any advantages later: my teachers were always moaning at me. Get him to play chromatic scales, sitting low and sitting high. It may become apparent to him then.