While waiting for adjudication. Often, the family would be given a notice to appear later in court for asylum hearings and released.

I assumed they were kept together somewhere for a short period, because I know we didn't send children to internment camps.

Just a guess.

The U S has been arresting illegal immigrants on the border for many years. I'm sure there's an established practice.

Unaccompanied minors were also sheltered. President Obama had to search for their families in their home country and would return them if they could do so safely. think someone posted a newsweek article about the abuse unaccompanied minors suffered. This was reported at the time and changes were made to get those children out of the ICE detention centers

Here's an article
[link:https://www.ajc.com/news/state--regional-govt--politics/immigrant-families-arrested-the-border-were-detained-together/t5CB9JzZBJx84EaiQ0AIOI/|

Before the Trump administration started separating parents and children caught illegally crossing the border, immigrant families were held together in two facilities in South Texas.

Human rights activists were outraged by the Obama administration’s decision to lock up the families, calling the practice inhumane and psychologically harmful. Immigrant watchdogs, meanwhile, argued the family detention centers helped deter illegal immigration and protect national security.

» From 2015: Photos from inside a family detention center

At the South Texas Family Residential Center in Dilley, Texas, women and children lived 12 to a building, complete with bunk beds. From their barracks, they could walk a short distance to a library, chapel, and medical and dental clinics. There was also a “city park” with a soccer field, volleyball court and playground. Children attended school there four hours a day.