There's an unwritten rule in horror: don't kill the kid or the dog. In general, audiences don't want to see it, so if a film is going to cross the heinous line, it has to make the death poignant and significant to the story. That's why the dog can often be seen climbing out of a crawl space, several scenes after seemingly perishing off-screen, whereas children tend not to be involved, at least not as the victim.