I'm sure some kids do. I wouldn't let mine watch it until they're 15 or so at least (it's not just monsters, there is sadism and misogyny in some episodes which I just wouldn't want them to see), but by the time they are it probably won't be cool anymore anyway.
I'm 20 and I still don't like watching Supernatural. Not just the violence, but the first season in particular is actually quite spooky. Makes me scared of the dark, no idea what it would do to more impressionable younger minds. I wouldn't let my kids watch it until they were over 15 for sure.