Ehhh...spoiler alert...
The way she is presented though in those initial scenes is in very surreal, almost dreamlike scenes in my opinion that when you're watching the movie the first time are kind of ambigious, and with the young girl hinting that she is the witch potentially (I have a huge theory about what this movie is ACTUALLY about, which is essentially the teenage girl going widdershins back in time and killing her family), but when you're first watching the movie you're not sure if its a dream, a delusion, the teenage girl doing this, an actual witch? At least that's the impression I got. I see what you mean though, it essentially states that there IS a witch and she is the one that stole the baby in the first few scenes. But who or what is the witch? I think it's the teenage daughter as stated before.