Why is any child still missing?
When the worlds photos are concentrated and searchable in Google Photos, Facebook, Instagram, why is any child still missing? It would be one significant upside to the panopticon: no more missing children!
Why do you think a picture is so likely to be posted? Not even thinking of murder victims. You won’t find pictures of hardly any kids in my extended family circle and their faces can change dramatically over relatively short periods of time.
A counter proposal to reduce missing persons: geo tracker always reporting on everyone and stations that detect “non-tagged” people and authorities then geo tag them. Now when anyone goes missing, you check their location history and cross reference it with everyone in that area. Still has issues with areas of no reception and of being terrifyingly orwellian. And bad actors will still figure out ways to work against the system. Just like they would with missing kids and photos - just don’t take or allow photos.
Consider that some teenagers run away from bad situations. I’m not sure that identifying and locating them is in their best interests.
So, who should control that great central database? In some countries, law enforcement would have a tendencies to blackmail people who are trying not to be found.
You are making an interesting assumption about the reach of such technologies. There are developing countries where you'll find thousands of children (say 5 year olds) who are never photographed (no photo exists).