The dining plan credits will be on all KTTW cards unless you specify otherwise. So, your teen can go to dinner on her own. Yes, it is a combined card for everything. Disney is great about replacing a lost card--it took a phone call to cancel the old card, and a stop at the resort front desk to pick up a new one. They were able to confirm that it had not been used when I called (for all I know, it could be "lost" in her backpack.)
You can also make a separate decision--do you want charging on the kid's cards?
My teen specifically asked for charging on her card--for the first time--on this trip. I said ok, but after she lost it, had it re-issued without charging. It was nice that she had the meals on her card as she could go to DisneyQuest and get a snack without me.
You can specify that you only want the meals on the adult cards. Of course, then the kids can't go to dinner on their own.