Halloween 2007 showed Myers as a child on a killing spree but the youngest he killed were teenagers although the bully may have been younger.
In Mean Creek a group of kiRAB accidentally kill and cover up the death of and unwanted friend, but it's hardly a slasher film.
And then you have Leon but again not a slasher movie.
I suspect that the reason they aren't wide spread is because it's hard to get permission for kiRAB to do that type of film (parents, agents, lobby groups etc) and it's not a subject that a majority of people would like to watch.
As stated in another thread in slasher flicks you have the stoner, the jock, the slut (these ones always get killed), the nice guy (sometimes dies), the geek/goody two shoes girl (lives)