Trump already floated the term in the media today -- "National Emergency". If he declares a national emergency and seals the border with regular Army troops (not National Guardsman), then the Mexican cartels are going to suffer because the porous border will shut down and make it hard to smuggle drugs and weapons. When it starts to sink in that business will suffer, either the cartels will wreak havoc on these people, or the Mexican government will step in with major force. Either way many of these people will suffer severe consequences -- Mexicans aren't known for showing mercy to foreigners. The cartels will just make them disappear.
Also, check the weather report. Hurricane Willa is going to hit central Mexico like a hammer -- these folks will most likely drown or die in the aftermath.