TENOSIQUE, Mexico — For years, Mexico’s most closely watched border was its northern one, which generations of Mexican migrants have crossed seeking employment and refuge in the United States. But the sudden surge of child migrants from Central America, many of them traveling alone, has cast scrutiny south, to the 600-mile border separating Mexico and Guatemala. :snip: Last year, Mexico deported 89,000 Central Americans, including 9,000 children, the bulk of the returnees coming from Honduras, Guatemala and El Salvador, officials have said. In the fiscal year that ended last September, the United States sent back 106,420 from those countries. So...