Yeah, I'm going to agree with Curly on this one. I vividly recall some of the creative writing stories I did in primary school, the vast majority descended into gore and murder mysteries by the time 'the end' (in lovely cursive script lol) rolled around... and I don't think I was a particularly morbid child. Just found it interesting as it was a bit 'taboo', so to speak - I wasn't permitted to watch M-rated movies so I'd write my own M-rated (or at least pretty hardcore PG lol) tales for assignments
Context puts things in a whole new perspective, really... although I can't recall a teacher ever setting a similar task when I was in school - more just 'Okay, this week you have to write a story about (eg) cruise ships' and I turn in some story about a murder mystery in which the captain is thrown overboard with no life-jacket, the sauna door is mysteriously locked, leaving clients to fry in their own juices and there are blood spatters everywhere I could find an opportunity...

I'd vouch it's as age-appropriate as it can be for kids of that age - I'm sure they all have vivid imaginations and even if they don't grasp the reality of 'murder', I'd bet it's fun for them to let loose and write about something a bit 'naughty', kwim? Probably gets better results than asking them to write about the family of ducklings down at the pond :/