IMO, I don't see why it would be bad to teach them about Paganism. How is it any different from teach them about any other religion?
Teaching them Magick, however, is a different story. I see nothing wrong with teaching them about Magick, yet actually teaching them Magick, is not an intelligent move. You see the difference?
Magick is intense and powerful and should not be taken lightly, even for adults, and for those who've practiced for years. So it, in itself, should not be taught to children, though teaching them about it, that it exists, what it entails, etc, I see no problem with.