Hmm... I like this idea, but can see why it's not for everyone so it would definitely have to be optional.
Kids usually have their own card, right? I was wondering how my kids were going to keep track of their cards since summer clothes are notorious for having lame or non-existent pockets. I think they might like a lanyard, especially if we made it a project before we left home, that they could decorate with beads or pins or whatever (nothing too fancy).
As for hole-punching, there are always those clear plastic nametag holders that are meant to go on lanyards. You could slip your card in there so no holes would have to be made, and it would be easy to remove for a non-lanyard event like formal night. I made lanyard nametags for our Sunday School staff at church and they work great (and were CHEAP to make).