The singer, who has Junior, 5, and Princess, 3, with ex-wife Katie Price, explained that he was careful not to let his kids see him in bed with the model.
He told Now: "I never let them see Elen and I in bed together, not ever.
"It was a big thing for me to go public with that relationship, and I was careful not to let the children see anything they shouldn't.
"As far as they were concerned, Elen was a friend."
The 38-year-old, who split from Rivas this month, said that he would invite her daughters Luna, 5, and Isla, 3, to his house when he had Junior and Princess.
He added: "When the kids were with me, she would come over with her kids and they'd all have fun together.
"I never let them be pictured out and about with her. I was careful to deal with it properly. My children always come first in everything."