I can't see blaming your son or daughter-in-law for the dog's destruction. It's not that they "didn't take care of" the quilt. It was exactly where it was supposed to be. Unless they knew their dog had a habit of chewing up things like that, there'd be no reason to suspect the dog would damage the quilt.
My first thought was, your son and his wife are all grown up now. If they'd wanted you to repair the quilt, they would have told you what happened back when it happened and asked you if it could be repaired. You said they're always asking for things. So since they didn't make the request, I wouldn't repair it. If they ever do ask, then put it on your list. Since they didn't ask right away, it can wait until it's convenient for you. And basic patching should be fine.