Well, If you rate it an 8, I am pretty sure every 10 year old that comes across it will definitely read it.
And technically, I don't think you can actually "kill" a zombie. Generally they are already dead. Or at least have already died and are now undead - one that is dead but reanimated. So in effect you are not truly killing the zombie but merely deanimating it, which is a family friendly activity that all ages should enjoy.