I don't know why I can't find an appropriate dictionary definition (it's not even in the full Oxford English Dictionary), but...
[Many child migrants] were coralled in [a detention centre].
Even though I can't find a definition, there are lots of matches for the highlighted search string, so the intended sense is obvious.
EDIT:
Ooops! My bad spelling (but given the "lots of matches" I found in Google Books, I don't feel too embarrassed! :) ...
corral (noun) Merriam-Webster
a pen or enclosure for confining or capturing livestock
corral (transitive verb)
to enclose in a corral
...which regardless of whether it's corralled or (far less common) corraled, still gets far more hits in Google Books than my completely mis-spelled version above.
A more recent term specifically used in the context of (riot) police packing protesters into a small, easily-controlled space...
kettle
Kettling (also known as containment or corralling) is a police tactic for controlling large crowds during demonstrations or protests. It involves the formation of large cordons of police officers who then move to contain a crowd within a limited area.
But no-one seems to have mentioned the obvious choice...
The US government confined a large number of child migrants at its detention centre.