A pioneering work from a visionary anthropologist, The Children of Sanchez is hailed around the world as a watershed achievement in the study of poverty--a uniquely intimate investigation, as poignant today as when it was first published.   It is the epic story of the Sánchez family, told entirely by its members--Jesus, the 50-year-old patriarch, and his four adult children--as their lives unfold in the Mexico City slum they call home. Weaving together their extraordinary personal narratives, Oscar Lewis creates a sympathetic but ultimately tragic portrait that is at once harrowing and humane, mystifying and moving.   An invaluable document, full of verve and pathos, The Children of Sanchez reads like the best of fiction, with the added impact that it is all, undeniably, true.