A Brockton mother of five fatally stabbed her 8-year-old son 50 times with a kitchen knife as part of a “voodoo” ritual, then went after his little brother when she thought she had “failed” with the older child, authorities said. “It came out ‘wrong’ with the first one and for that reason she had to move on to the second child,” assistant Plymouth District Attorney Jessica Kenny said of Latarsha L. Sanders, now held without bail for the murders of her two youngest children.