A mother is upset that last week her five-year-old son was allegedly handcuffed to a chair after throwing a temper tantrum in his Queens kindergarten class. The incident occurred last week at PS 81 and Jasmina Vasquez said her son Dennis Rivera was terrified. Rivera, who is quite large for a five-year-old at 68 pounds, reportedly was having a fit and knocking things off desks, when a school safety agent cuffed his hands behind him......