Do they know what in tune sounds like? Rather than using a tuner, use a drone? To teach what “waves” are, I play a bass guitar while tuning a string and the waves are pretty obvious when I do that. I teach them to listen to the beats and how they get closer together when it gets farther away from in tune, etc.

Maybe try and work their ears? Just throwing something out there. Not trying to be snarky at all. You asked for help, and maybe this angle will help.

Best of luck!