A US government study released yesterday shows that parents need to worry about more than stranger danger: Youngsters are more likely to be kidnapped by an acquaintance.  The most common kidnapper of children 17 and under, listed in 49 percent of cases, is a member of the family, according to a report by the Office of Juvenile and Delinquency Programs.  The next largest category was acquaintances (27%), followed by strangers (24%).  In acquaintance kidnappings, often by boyfriends or ex-boyfriends, 23% of female victims also suffered sexual assault, compared with 14%