Our narrator is twelve-year-old Jack. Since we see everything from his perspective, and he's right there in the thick of all the action, the story unfolds as Jack is seeing it. We only know what he sees, and we only get his interpretation of events.
Because he's only twelve, Jack doesn't always catch onto things as quickly as an adult might. For example, it takes him awhile to piece together all of the events, because he's still learning those important critical thinking skills that you've probably heard so much about. This limited narrator creates a kind of cool effect, where we see what Jack sees, but, because we're standing over his shoulder, we can also put together pieces of the story that he can't quite.
Admit it: didn't you start to suspect that the old ladies were being poisoned before Jack? (But did you figure out who was doing it?)