Richard Latham, the prosecuting lawyer, said Philpott and his wife, Mairead, did not intend the children's deaths but the scheme went "horribly wrong," The Sun reported.
Philpott, 55, an unemployed baker, has fathered at least 17 children, including the six with Mairead and four with Lisa Willis, 28. Latham said he was trying to get custody of Willis's children after she moved out of the trailer she occupied in the driveway at the Philpotts' home in Derby.
Philpott and Willis had a court hearing scheduled the day of the fire. Latham said Philpott had been making threats about two weeks earlier and told police Willis had threatened his life.
"He told people he had a plan up his sleeve and that she wasn't going to get away with it -- watch this space," Latham said.
The Philpotts wept as Latham played tapes of the calls they made to the emergency line as the fire raged. At one point, Philpott, saying, "I can't listen to it," tried to leave the court but was stopped by officers.
Latham described Mairead as a "lapdog" willing to share her husband with another woman and to go along with his arson plan. The couple are charged with six counts of manslaughter, and a friend, Paul Mosley, also is on trial.