As a question of fact, it's just "true" that parents often don't know what's best for their kids. They don't know which medicine is best for an infection. They don't know which math topic is best learned next. They don't know a lot of things. Nobody knows everything, or even any significant fraction of all that there is to know.
Teachers don't know everything either, but there is such a thing as subject-field knowledge and a good teacher has that.