Simply disabling downloads is not enough, they would need either an effective DRM system, or to kill off third party downloaders such as youtube-dl. I'm sure the RIAA would love to be able to go after youtube-dl on child porn charges instead of just cppyright charges.
They could attempt to implement a content-ID system to detect re-uploads. I suspect this could work better for child porn than copyright, because you will not have adversarial parties trying to claim that you uploaded their child porn.
The difficulty in such an approach is that the mere possession of child porn is a crime, which makes developing and maintaining such a technology far more difficult and expensive (even with the law enforcement support that would be necessary to make such development legal).
The more insideous effect is that putting this requirement on pornhub also puts requirements on every platform supporting user generated material. In order to avoid getting caught up in the child-porn minefield it creates, platforms would need to censor all porn and porn-adjecent content.
