What happens when you set a tee high and inside?
Try that and tell them to hit the "inside of the ball". Position the ball so that a Logo would be at the 9 o'clock position on the ball for a right-handed hitter; 3 o'clock for a leftie. That should force their hands inside. I think approach...
I agree with @jamesd1628
Try working on "pulling" the knob with some bottom hand drills. My son had the same issue. We really work on "bottom hand" drills before I have him hit. Telling the batter to use your hands isn't popular, but I believe your options are to teach to pull the knob with...