A Billerica man charged with raping a 6-year-old girl at his girlfriend’s unlicensed day-care center has been ordered held on $100,000 bail. Michael Martel pleaded not guilty Monday in Lowell District Court to charges including multiple counts of aggravated rape and rape of a child with force as well as indecent assault and battery of a child under 14. Prosecutors allege that the 42-year-old Martel would get the girl alone at the day-care center at the home he shared with his girlfriend and rape her. Martel was arrested Friday after the alleged victim was interviewed. He denies the allegations.