en.m.wikipedia.org/wiki/COPINE_scale
If look on this link, you will see that it is illegal to collate images of other people's children in this country, even if they are clothed, if the scale and context indicates they have been collated for sexual purpose.
I've worked with and around sex offenders for the majority of my career. Those saying it does the child no harm for their image to be used by offenders, are wrong. If the child finds out, they have to live with not knowing who has seen the image, when, what they've done with it. Wondering if someone is looking at them oddly, because they have seen the image.
If the child doesn't find out, they may not be directly upset, but they've been degraded, defiled without their consent. That matters.
Then there's the question of whether using these images contributes to fantasy, and escalation towards contact offending (jury's out on that one).
For me, what it comes down to, is this. I've met these men. I know what they do. I don't want them wanking over my two year old while trading fantasies about torturing and killing him.
Say that's dramatic if you want, but I assure you I know what I'm talking abou.