Most Western nations already have agencies that determine the fitness of parents regarding child-rearing. In the US, CPS has the authority to remove any child from "unfit" parents. If the OP's suggestion is adopted, CPS-like agencies will no doubt be the enforcers.
Your comment is quite valid in questioning the practicality of enforcing such a legislation. The child protective bureau will need to be enlarged dramatically to patrol the internet and enforce the new rules. Special judiciary will need to be set up to process the custody hearings arising from CPS enforcement. New surveilance powers will need to be ratified to empower CPS to effectively monitor the internet. Like the "drug war" and "war on terror," the cost of war on child photos will be born mostly by the poorer class of our society.
The political question is whether the lower class can bear the additional cost without revolting.
In my state, CPS has a horrible track record of protecting children. The last thing we want to do is have them coming after parents who post pictures of their kids on social media.
CPS-like agencies around the world have poor records, regarding child welfare. The nature of bureacracy is that each department carve out fiefdom within the government and its primary purpose becomes maintaining that fiefdom. In essence, the departments have no interest in solving the problem, only expanding their mandate, so their funding increases. I hope some CPS spook is not considering the OP's concerns and drafting a proposal.
"The nature of bureacracy is that each department carve out fiefdom within the government and its primary purpose becomes maintaining that fiefdom." You nailed it!!!
I agree with you, @soo.chong163. Thank you for posting this great comment!