Read this twitter thread from an immigration lawyer. The Office Of Refugee Resettlement is essentially a jailer that takes in minors who cross the borders by themselves and discharges them to family members or other close relations. They’re “missing” in the sense that these families aren’t picking up the phone when the ORR calls to follow up. Because what immigrants wouldn’t want to talk to Trump’s government. The twitter thread above actually says how this misplaced outrage can backfire terribly: people might demand the ORR be given broader powers to track down the children that have left their control, and we really don’t want that.
Edit: because that lawyer is having a lot of trouble with people who aren’t getting this: the children missing are not those who were separated from their parents by ICE, they were unaccompanied minors.