My dd 4 has been watching Sleeping Beauty for quite a while now, and is not afraid of Maleficent. We talked about her when she first came on and how she's mean and nasty and that the good ones (Prince Philip and the fairies) will win. I think she's not bothered by Maleficent (or Melissafant as dd calls her!) because she doesn't look at all real, and she's working with magic spells.
Snow White, on the other hand, we still skip the scenes with the huntsman. It's just too real looking and too heavy to talk about for a 4 yr old.
I agree with other posters that it depends on your child, and depends a lot on how you approach it before watching. It's definitely not worth it to set them up to be afraid of Disney characters!
Have fun,