For those who are lost, pogs are cardboard coins. People contribute equal numbers of them to a stack or pile, and then take turns throwing heavier coins called slammers at them. You keep any pogs you flipped over and the winner is whoever flipped the most. They were a big fad in America in the 1990s. My setting is the 90s, but Japan. I have room to fudge the dates by a few years, but not whole decades.
I know pogs came from a Japanese game called menko, but pogs are specific. They're circles of a uniform diameter (there are standard pog tubes to carry your collection in), they're hit with a slammer instead of just another pog and so are probably lighter than menko cards, and I think menko comes in neat sets whereas pogs tend to be a haphazard collection that changes with every game played. Pogs were regularly given out individually or sold from a bin of mixed ones. Pogs are played with a stack/pile, not just one card each like menko, and collecting pogs is half the fun. Most importantly for my story, pogs were a popular fad. Everyone played/collected them and then no one did.
So far I've read the Wikipedia on pogs and menko, and and skimmed their talk pages. I read another wiki on how to play menko. I've been through the "Japan (misc)" tag here.
Search terms: "did they have pogs in japan" "pogs japan" "pogs japan -com" "remember pogs" and just "pogs".