Not really in custody are they.
Check it out:

With communities across the country pushing back on plans to locate young illegal immigrants in their area, the federal government is looking towards military bases to house the thousands of children streaming across America’s southwestern border.

The Department of Health and Human Services announced yesterday it was no longer considering a Federal Emergency Management Agency facility in Anniston as a temporary housing location for undocumented children. On the same day, the Department of Defense announced a fourth military base will be opened as a shelter site.