Do not misbehave. Do not sit on the floor. Do not share your food. Do not use nicknames. Also, it is best not to cry. Doing so might hurt your case.

Lights out by 9 p.m. and lights on at dawn, after which make your bed according to the step-by-step instructions posted on the wall. Wash and mop the bathroom, scrubbing the sinks and toilets. Then it is time to form a line for the walk to breakfast.

“You had to get in line for everything,” recalled Leticia, a girl from Guatemala.

Small, slight and with long black hair, Leticia was separated from her mother after they illegally crossed the border in late May. She was sent to a shelter in South Texas — one of more than 100 government-contracted detention facilities for migrant children around the country that are a rough blend of boarding school, day care center and medium security lockup. They are reserved for the likes of Leticia, 12, and her brother, Walter, 10.

The facility’s list of no-no’s also included this: Do not touch another child, even if that child is your hermanito or hermanita — your little brother or sister. Leticia had hoped to give her little brother a reassuring hug. But “they told me I couldn’t touch him,” she recalled.

In response to an international outcry, President Trump recently issued an executive order to end his...