By Christopher Walljasper and Steve Gorman CHICAGO (Reuters) – An 8-year-old boy suffered a severed spinal cord from a gunshot wound to his chest in the July Fourth attack on a Chicago-area parade that left seven dead, a family spokesman said on Thursday, with his twin brother and mother also wounded. Cooper Roberts was one of the most gravely wounded among dozens of people who survived gunshots and other injuries in the Highland Park Independence Day assault, underscoring physical and emotional devastation that goes beyond the lives lost in such violence. He was listed on Thursday in critical…