Without bothering to look at the link, I'm pretty sure that paedophilia (insert all American/non-classically trained spelling variations here) is the abuse of pre-pubescent children.
I'm not trying to defend Thomson in the least when I say that these girls would be post-puberty and sexually aware. It's alleged that more girls were involved and Thomson was prosecuted only on the basis of abusing the two girls. Obviously these images would have been circulated fairly freely and some of the girls would have seen it as a bit of a laugh. It's likely that many boys would have seen the images too. A quick scan of Facebook will reveal, if anybody was unaware, just how sexualised modern teenagers are.
But however sophisticated these modern teenagers think they are, that's what the law is for: to protect minors against the likes of Thomson. In my line of work, teenagers passing explicit images, by phone or otherwise, are deemed to be at risk, and I would be obliged to report any instances of this.