Many coaches work hard to get their swimmers to improve their swimmers’ distance per stroke (DPS). Without a very clear understanding of ‘why’ you should have a long stroke, a swimmer will, often, slowly shorten their stroke. The ‘why’ makes the process easier for consistent application.
I believe the trick is to get the swimmers to understand the underlying reason for making a long stroke so that the principle sticks with them. The ‘why’ is as important as the skill.
One way of teaching swimmers to get it is to use a simple analogy. I use this example: imagine you are walking along the beach by the shore. Someone has been walking before you but you can’t see them anymore. For fun you’ve decided to try to step in their footsteps. As you carry on, placing your foot in each impression, soon the footsteps become harder to reach and then much too far apart to reach. The steps got longer and longer, and soon you have to leap forward to try to reach each step. Then you find that you are bounding very far to place your feet in the footsteps.
What has happened? Did the person you were following all of a sudden get longer legs? Of course they didn’t. They started to run.
Swimming fast requires long strokes because it is like running. When you run, in some part of the stride, you are flying through the air. In the 19th century people didn’t believe this was possible but strobe light photography proved it. Swimmers too glide between their strides/strokes and their strokes are very long when they swim fast. In particular the fastest swimmers in the world.
Two teaching points (and likely many more) can be squeezed out of this analogy; when you run, your strides are further apart and conversely, if you tried to run in walking footsteps it would soon become too short a stride to hit each foot placement.

Training specificity requires repetition to adapt correctly. If you repeat a long stroke you will get strong and proficient at that length. When you teach yourself to repeat a long stroke you are preparing for swimming faster in anticipation of a quicker tempo. What you practice is what you get good at.
There are many things to teach young swimmers but without distance per stroke your group will be far behind the pack. I suggest taking them to the beach to learn to swim fast or at least an imaginary beach!