Generally, children who access abusive images on the internet will be resilient enough to deal with the shock of what they have seen and put it down to experience. I remember other pupils passing round such material in maazines when I was a teenager and most of us were shocked and amused at the same time, but it was not a priority for us and we could leave it in its place. It's the isolated children, or those who do not have guidance to compare what they are seeing with what is more appropriate, that I would be concerned about. Many satisfy their curiosity and move on - very few return to it again and again, and when they do so in the company of peers who react appropriately, they won't come to much harm. It's the remaining (very) few who develop an inappropriate interest, or even worse, get hooked by online predators.
It appears this 14 year old did not come into the 'concern/high risk' category when he was assessed, which enabled the judge to take the action he did - he wouldn't have done this independently, but would have considered several reports, including the actions from a public protection panel, whose members would have discussed the incident at length, and heard or read the views of the parents of the child who reported the abuse - well done that child - she knew at 4 that she should tell her parents so they could protect her. She's probably a bit wiser than them for leaving her with the boy. I'm not saying 14 year old boys are a risk - generally, when they have a close bond, as with siblings, this brings a degree of protection for the child.