An 18-year-old Malden man has been charged with raping a child for whom his mother baby-sat, authorities said. Robert Guevara pleaded not guilty at his arraignment Wednesday in Malden District Court on two counts of aggravated rape and was ordered held on $100,000 cash bail, Middlesex District Attorney Gerard T. Leone Jr.’s office said. Guevara is accused of assaulting the child repeatedly between February 2011 and December 2012, while the child was under the care of his mother. He is due back in court Jan. 29. Leone’s office is not releasing the child’s age.