I screened this film at my University's Cult Film Society and it received a great, yet split reaction from the audience.
The film does question you on a number of levels, and also is one of the only films that truly emphasises the classical meaning of horror, that of loss. In this case a loss of innocence and self control.
The film is definitely worth buying if you're interested in exploring the question of the title, essentially it asks the viewer: could YOU kill a child if the situation depended on it? In our case many decided that they would kill the children in order to save themselves and their family.
We watched the Spanish language version with the English subs because the film was initially released in Spain and we wanted to experience it in its original form. However the two leading characters' lines were recorded in English and then dubbed into Spanish, therefore the majority of the film was originally filmed in English. I recommend watching both the Spanish and English dubs (both included on the disc) in order to get a more rounded feel for the movie, as at some times the English subtitles made the film seem more like a novel and as a result the completely English audience may not have felt the same emotion for the characters as they would have had an English audio track been playing. Also the subtitles change a tad too quickly before you can read them all on some occasions.
A number of reviews have criticised the slow pace of the film. This film pre-dates the fast-paced norms that Western audiences got used to during the slasher boom of the 1980s.
Essentially the interview with the film's cameraman (one of the disc's special features) says it all; they weren't trying to make a horror film but a psychological thriller, and they certainly succeeded.
The film is not boring, however it does take a good 30 minutes or so to get going, but if anything this heightens the shock-value of the final 2/3 of the film.
The extra features are fairly interesting though not particularly vast. It would have been nice to see a documentary or perhaps an analysis of the film by a scholar, comparing it to other films of its time, but what with Who Can Kill A Child being a fairly obscure film I doubt anyone of any notoriety would be interested, despite it's interesting subject matter and being an all around good film. To be honest, I was surprised that Dark Sky managed to interview two of the films key creators, so good on them for the effort.
Overall I didn't give the product 5 stars because of the few quibbles I had with the disc with regards to presentation/etc, however the movie itself is a good 7.5/10.