Kevin Federline says he has been on full-time daddy duty since getting custody of his two toddler sons.

"I spend most of the day chasing my kids around the house," Britney Spears' ex-husband tells In Touch Weekly magazine.

"Their needs define my schedule every day."

Federline has had sole custody of Sean Preston, 2, and Jayden James, 18 months, since early this year after months of bizarre behaviour by Spears. The couple, who wed in October 2004, divorced last July.

Spears is now under the court-ordered conservatorship of her father, James Spears.

"It is so very important to me that my children will always know and never question, that no matter what happens, they have a mummy and daddy who love them very much," Federline, 30, tells the magazine.

Last week, the 26-year-old pop star was ordered by a Los Angeles court commissioner to pay Federline $US375,000 ($A410,000) to cover his legal fees in their child-custody dispute.

Federline also has a son and daughter with ex-girlfriend Shar Jackson.

"I will always love the mothers of my children, and I will always be there for both of them," Federline says.

"I believe that any good energy I put out there brings more good energy into my own life. The relationships with my kids and family will last my whole life and are the most important to me," he says.