It doesn't matter why the child is in the video. The pedophile only cares that there is a child. Videos uploaded with children in them will be exploited, regardless of whether they are legitimate.
I feel like you have missed the point. YouTube is actively helping people find this content and making money off of it.
The article claims that there is nudity (via timestamp comments). Separate models could be used to score the video, where a child plus any other potentially sexual content (genitalia, nipples) withholds the video. A withheld video can be appealed (for scenarios such as "breastfeeding tips" or what-have-you), much like copyright strikes, which have an existing process.
Obviously the model might have problems discerning between a 17 and 18 year old, but it would catch the most egregious content.
As long as child is not performing some sexually suggestive stuff on youtube video ("popsicle challenge", wtf) it is OK, probably.