There’s some sharp dialogue in “Cooties,” the sort of grisly fun that most zombie movies these days aim for, but it’s not quite enough. The idea is made pretty obvious by the title: It’s kid zombies. The film’s clever opening sequence gives us the backstory about tainted chicken nuggets on the school lunch menu. From there, the school’s staff has to figure out how to combat the virus that only targets pre-pubescent blood. The cast is familiar, even if you wish they were together for something a little more high-minded—Elijah Wood, Rainn Wilson, Jack MacBrayer, Allison Pill—but ultimately, the sizzle the first 20 or 30 minutes gives us is too hard to sustain. If the word “zomedy” did not previously exist … you read it here first. Unfortunately, the silliness at play outweighs the knowing wink that this is all silliness.