I will preface this review by saying that I've only watched 3 episodes. I will come back and update this review when I have watched more / the rest.
This is a very well made and compelling series about serious issues. The settings, the cinematography and the acting is superb.
The warning here about violence is not sufficient in my opinion. This is a very violent series. Maybe not by Korean standards but from the point of view of other films.
This is not a series to watch with young children, because of the violence and the mature issues that are dealt with. It should be acceptable for mid to late teenagers, assuming they can deal with difficult issues.
The violence and the shocks here are too much for younger children IMO.