A cue that seems to work is to tell the student to push against their big toe of their back foot as their rear hip triggers.
Well, maybe.
But indulge me for a second, and lets see if we can both learn something:
Stand opposite one of your hitters. Have her load. Grasp her shoulders firmly. Then tell her to launch the swing and rotate her hips.
Typical young hitters (or older mediocre hitters) will basically push with the rear hip, turn the rear knee, and turn the rear foot on the ground. And it is not at all difficult to hold their shoulders in place.
Really good hitters will tend to use their upper legs, hips (especially the front hip) and forcefully enter rotation. Their rear foot will start to roll up onto the toes even before you release their shoulders. And you WILL have trouble holding their shoulders in place.
I am NOT picking on Epstein. I have met him personally at an NFCA coaching convention, and I liked him a lot. I'm sure he is good working with kids. That said, when I look at hitters on his web site, or for that matter, who have been working with his instructional materials, they are instantly recognizable. They tend to almost universally have a LOT of weight on their back side, so if that isn't his message, his message isn't getting through.
It is obvious that Epstein considers the number one problem he faces to be lunging.
That has not been my experience, but I respect his opinion. I just think he has too much emphasis on curing this problem, which I don't find that prevelant. In fact, I am far more concerned with getting weight to the front side. Failure to do so is a much bigger problem than lunging for female hitters, in my opinion. Of course, once the weight is forward, the front side has to firm up and rotation has to occur.
From what I know, Epstein does a good job with getting hitters to understand the importance of getting on plane, and leading with the hips.
But I feel he does nothing to teach hitters how to use their shoulders, arms, and hands. Certainly, resting the bat on the deltoid precludes much emphasis here, and he seems to say as much on his videos. That is a huge limitation for me.
The fence drill does not look at all like the swing I am trying to build in hitters. I undersand that Epstein wants hitters to stay "inside the ball." But this is the wrong drill to accomplish it, in my mind. Just as easy to disconnect by being too far inside and pushing the hands forward as it is to diosconnect in any of dozens of other ways. And the way Epstein implements the fence drill, the hands DO seem to push forward.
Do yourself a favor, and try the deal holding the shoulders, then think about what you observe when you do it.
Best of luck with your young hitters. You have a crucial job in their development. I can tell you for a certainty that by the time they are college aged, it is very hard to change them. To a great extent, those of us who work with older hitters are dependent on what you do years before.