Five Lowell residents, including two children, were seriously injured in a head-on car crash in Methuen on Sunday, according to the Essex district attorney’s office. The accident happened when a 1999 Mitsubishi and 2007 BMW crashed in Methuen, a spokeswoman from the district attorney’s office said in an e-mail. The Mitsubishi driver, a 37-year-old woman from Lowell, had her two children, age 6 and 9, and her mother in the car, the district attorney’s office said. The BMW was also driven by a 37-year-old woman from Lowell. Authorities did not say exactly where or how the crash happened. All five people involved were taken to various hospitals with serious injuries, the spokeswoman said. No files had been charged as of Sunday night, she said.