Charlie Lau Jr referred to the top-hand as a swing poison. Why? Because he perceived that many had a tendency to push the handle forward at swing launch ... and when used that way the top-hand is pretty much a swing poison.
On the other hand, in my opinion anyway, if you promote a solid top-hand path, then the top-hand is far from a swing poison, but is instead an incredible gift.
How do you teach the solid top hand path? Are there specific drills or posts you can point me towards? Jr's book was transformative for me as a hitter. While I try to not give too much direction to my 9 year old, when I do, it is going back to the principles in that book.