Mick Philpott, 55, and his wife Mairead, 31, were in custody and being questioned by detectives about the blaze, The Daily Mail reported.
The couple had appeared grief-stricken at a news conference two weeks ago during which police said fuel had been poured through a front-door mail slot and set afire.
The children were asleep in their beds upstairs in the semi-detached house when the fire broke out early May 11, authorities said.
Assistant Chief Constable Steve Coterill said Tuesday the arrests "are just one step further in the investigation."
"I suspect there may still be people with crucial information who have not yet come forward to speak to us. ... It is absolutely vital that if you know anything you think could help us, come forward now. Do not wait any longer. It is important that we find justice for these six young children."
After the fire, The Daily Mail said, suspicion arose that someone had a grudge against Philpott, who was widely known after media coverage as a father of 17 children by five women.
Lisa Willis --- who had lived with Philpott, his wife and children he fathered who were born to both women -- was initially arrested on suspicion of murder in connection with the fire, along with her brother-in-law Ian Cousins.
Willis, 28, and Cousins, 38, were later released without being charged.
Willis had moved out of the house where the fire occurred this year after relationship trouble with Philpott, and her five children by him moved out as well, The Daily Mail said.
The Guardian identified the children killed in the blaze as Jade Philpott, 10, and brothers John, 9; Jack, 7; Jessie, 6; Jayden, 5; and Duwayne, 13, who died of his injuries in a hospital.