BOSTON, March 16 - For the first time in two centuries, a generation of children in America may have shorter life expectancies than their parents, according to a report that contends that the rapid rise in childhood obesity, if left unchecked, may shorten life spans by as much as five years. The report, to be published on Thursday in the New England Journal of Medicine, says that the prevalence and severity of obesity is so great, especially in children, that associated diseases and complications - like type 2 diabetes, heart disease, kidney failure and cancer - are likely to strike...