"Until their skill in a subject surpasses yours"

Which for all but one or two subjects, it will.

Look I'm not saying parents can't ever teach their children anything. What I'm saying is that parents need to know their place and role, and they can do much more good by being emotionally supportive and in general creating an environment in which children can and want to learn, than by being yet another instructor who's trying to cram ever more things into the child's head (much of which will be different from what their school or team coach is teaching them anyway).

This is absolutely correct. I learnt this fast with 1st/2nd grade homework - I do more harm than good by actively participating. Sure I know what 32-29=3 but I do not know how that is being taught in the classroom and if I start sticking my oar in it goes sideways quickly. It's far more constructive to be supportive in exactly the way you mention.