Whether or not to teach your own child is a matter of personality and temperament for both child and adult. I wont teach my own boys until they are around 10. Until then I would bring them to someone else to "have fun". However I have played ninja turtles with my 5yo. That lasts about 15 minutes then he's on to something else. Professionally speaking I usually don't take children under 6. It just never seems to work for me. My own temperament I guess.
I agree completely with this. My wife is a black belt candidate, one of my daughters reached 3rd Geup before moving away, and my son reached 4th geup before moving. I've enjoyed training with them all.
But I've always avoided being their primary trainer, and I've never had anything to do with their promotions.