A Mexican company announced yesterday that it would undertake the largest environmental cleanup in the nation’s history in Torreon, 500 miles north of Mexico City. But many say it’s too little, too late. Pollution from the world’s largest silver refinery in the town has caused severe lead poisoning in thousands of children, and despite studies from as early as 1981 indicating a serious problem, nothing was done about it until now. The new $36 million plan is to evacuate a 20-block area surrounding the plant, demolish 393 homes, bury lead-tainted soil under asphalt and dirt, and establish a $6 million fund to help sick children. The scale of the project rivals the Love Canal cleanup near Niagara Falls, N.Y., the most extensive soil containment project in the U.S.