My ds is mad about Pokemon but has never had any interest in the cards and (AFAIK) neither have his friends.
The Pokemon cartoons are usually on Cartoon Network. People often download them to Youtube too.
The basic idea is that a Pokemon trainer (who is often a child) travels around with his Pokemon. The Pokemon battle with the Pokemon of other trainers.
Most of the Pokemon are specific types, eg fire types, water types, ground types, ice types etc. It works a little like the game 'Scissors, Paper, Rock" where certain types are stronger against other types so:
Water pokemon beat Fire types, but Electric types beat Water types.
It's all very complicated tbh. My Ds loves the Gameboy and DS games where you catch Pokemon and train them up into a team.
There will be lots of other children who aren't interested in Pokemon. I would say that the children who are interested at your Ds' age are almost certainly the ones with older brothers and sisters who like it.