'Scuse me for posting here, but I just wanted to say Nothing ventured ....
I started teaching my older son when he was about 7 or 8, because I couldn't find a teacher, but we had the piano and he was keen to start. He did the prep test, more to let me know if I was doing anything wrong, rather than to give him exam confidence, and when I thought we'd covered everything thoroughly for Grade 1 he took that, and passed with 118 (he was 9 by then). Could he have reached a merit with another teacher? Possibly/probably. Did he mind just missing it? Not at all, fortunately
At that point I found a teacher for him and was happy to hand him over, because although I knew that I could help
him with the next level of scales and pieces, I didn't want to teach
them to him, and wasn't confident about teaching the aural and sight-reading skills correctly. I also worried that if I was teaching him we might end up just concentrating on exams.
I also started my younger son off (fond memories of Yankee Doodle), and that worked well, because when I asked the teacher if she could take him on too, she hesitated until she realised that he wasn't a complete beginner, as she didn't like teaching young children from scratch (I think he was about 7).
The only other thing I would add (which has been mentioned by others too) is, don't let every practice become a lesson (unless he prefers it that way!
). Mine preferred me to wear my not-quite-so-involved parent hat when they were practising.