This is a much deeper subject. What is defined as child pornography is different and has different exceptions based on what is the image and how it is used. By your definition a doctor conducting research by taking pictures of children's genitalia where that child has a medical condition could be construed by some as child pornography. Or if a parent innocently takes a funny picture of children in the bathtub playing like many parents did in the past that could be construed and child pornography. Or if a teenager takes a naked selfie of themselves and sends it to their boyfriend/girlfriend who is also under age both have trafficked in child pornography. In the US there is also been cases where there has been virtual child pornography cases where the courts have ruled that only an actual minor can be considered in the trafficking of child pornography. Also, most states and the federal government allow for the accidental viewing or downloading of child pornography but is also very specific depending on the jurisdiction on what needs to be reported and what doesn't.
Justice Stewart wrote:
"I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that." So there is always questions when it comes to pornography.
Also you have to remember some jurisdictions like Australia has much stricter rules on pornography in general than other jurisdictions. And what you are quoting from is a worldwide document.
