Just days after being initially charged with prostituting a 15-year-old girl, a Malden man and Lynn woman were indicted by a grand jury. David Minasian, 24, of Malden, and his girlfriend, Madonna Say, 23, of Lynn, were each indicted on a charge of sex trafficking of children, according to court records in US District Court in Boston. They were arrested April 8. Minasian found the 15-year-old walking the streets of Chelsea in July and took her on a road trip to Florida, where he plied her with alcohol, heroin, and painkillers, and first began to sell her as a prostitute, according to an FBI affidavit. Minasian and Say later allegedly posted online escort service advertisements featuring the girl, and rented a room at the Hyatt Hotel in Braintree.