The main factor was already mentioned: blueskins and anti ngo's harassing models, site operators, and card companies, so that the business became unsustainable. A few times, actual criminal charges were filed, but the purpose was just to disrupt the business, rather than discovering any illegal conduct, as shown by how few did stick.
Crypto currency should have brought back those sites if the money was the only issue, though. And the increasing familiarity and ease of use of the hidden Internet (dark web) should also solve many of the anonymity problems which led those sites to be easy targets, both from the supply and the demand sides.
That leads me to suspect other forces have entrenched the decadence of dedicated NN child modeling sites.
The first is the huge increase in availability of kiddie pics and videos from celebrities and professional fashion models through social networks. Every child actress, singer or Toddlers and Tiaras participant has her YouTube channel, Instagram, Twitter and Facebook. It is much easier to get many more visual materials from famous girls than used to be.
And conversely, many amateur girls post a lot of visual materials too, by themselves, to those same sites. So it is also trivial to access those and download those, for free.
Now yes, the sort of quality which NN child modelling sites had us used to is probably gone forever. But with social networks we have a substitute that is obviously good enough for most.