Carlos holds his bare feet, cut and bruised from the harsh concrete floor, as he sits in his cell in the Tonacatepeque juvenile detention centre, just outside El Salvador's capital, San Salvador.

The 16-year-old serving a seven-year sentence for murder in one of the world's most dangerous cities speaks quietly and without making eye contact.

“My mum just got up one day and left for the United States. I was nine years old,” he says.

Carlos belongs to what the United Nations has called the 'lost generation'. Across El Salvador, Honduras and Guatemala, Central America's so-called Northern Triangle, hundreds of thousands of children live without their parents on the streets or in care.

Those parents who haven't been killed or detained left their country in search of a better life, joining waves of migrants northwards. Donald Trump's pledge to shut them out using a "big, beautiful wall" has seen the largest ever 'caravans' march towards the US.

The seemingly intractable row over the wall's funding ended abruptly last month when Mr Trump lost patience and declared a national emergency to start work on the pet project that riles his base and embitters his rivals.