My kid didn't understand until the vurtual items he bought with his hard earned money, about 80usd, vanished when the company shut down. It was a brilliant educational moment. Alas Minecraft is probably good for another decade!
The problem isn't virtual items, everyone pays money for non material experiences/services - and on a long enough timescale even material things wear down/break.
The important thing to teach a kid is if the value they derive from buy a skin eg. and whatever it brings to them (happiness, bragging rights) is more important that opportunity cost of paying for that.
If the cost is always zero (daddy pays) - then the kid will not stop to think about it ever.
Something about telling a kid that the iron is hot vs the kid needing to burn himself to learn.