About four decades ago, most of the children S.S.I. covered had severe physical handicaps or mental retardation that made it difficult for parents to hold jobs — about 1 percent of all poor children. But now 55 percent of the disabilities it covers are fuzzier intellectual disabilities short of mental retardation, where the diagnosis is less clear-cut. More than 1.2 million children across America — a full 8 percent of all low-income children — are now enrolled in S.S.I. as disabled, at an annual cost of more than $9 billion.