That said, outside of the merits of this approach, I am dubious of any actual implementation given 2 points.
1) Protecting the youths will always be a leaky bucket. With disadvantaged youths possibly more at risk. Those exposed to non-compliant parents ("cool" parents who are ok with sharing unsuitable content) or lacking strong parental involvement, likely won't benefit a great deal from any implementation.
2) Anti privacy social networks stand to gain the most from targeting ads utilizing signals from most child safety acts. They also might be able to reduce some costs from moderation if they can make it someone else's problem. I'd argue the net social impact from these social networks is likely both more normalized and strongly negative for our youths than any smut.
On the balance I'd say we are better off investing our energy in other places.