The Palestinian Commission of Detainees and Ex-Detainees Affairs announced that over 50,000 Palestinian children were detained by Israel since 1967.
Where did they get this number from? They made it up.
Oddly, the number seems to have gone down. In April, the very same commission announced that Israel had arrested (not just detained!) more than 53,000 Palestinian children since 1967.
A different group, Military Court Watch, sent a report to the UN claiming that 95,000 Palestinian children were incarcerated (not merely arrested or detained!) since 1967. And that report was in 2015!
Given that there are never more than a couple hundred prisoners under 18 at one time, that is a neat trick.
This is similar to the "750,000" or "million Palestinians" supposedly imprisoned by Israel since 1967, another totally made up number that I thoroughly debunked here.
