As for their origin further south . . . exactly how do tens of thousands of children manage to travel the length of Mexico - a country notoriously sensitive about its own border - without the connivance of the Mexican government? I mean, the shortest possible route through Mexico from Texas to Guatemala is pushing 900 miles!
That's been bugging me too, and I'm leaning towards Makattak's explanation.
As a bonus, La Raza is telling the US how we suck and are racists, but somehow also ignoring Mexico, who could have been helping all these people as they crossed their Southern border (helping as in humanitarian aid, vs helping them get across Mexico's Northern border, so that Mexico can then also chime in about how we suck and are racists).
It continually cracks me up how the US is always the fall guy for our terrible immigration policies, when 95% of the other nations on the planet have incredibly strict immigration laws that make us look like we're an open borders country.