Kevin Federline fights Britney Spears for custody

Kevin Federline has enlisted the help of an ex-Israeli commando as he fights for the custody of the two sons he fathered with crumbling pop princess Britney Spears.

K-Fed's lawyer, Mark Vincent Kaplan, dispatched security expert Aaron Cohen to hunt down and subpoena Spears' assistant and cousin Alli Sims.

Also in Cohen's sights during the a pre-daw raid was Spears's producer, Jonathan (JR) Rotem.

K-Fed’s camp wants both quizzed under oath about his ex-wife’s alleged erratic parenting practices.

Cohen finally tracked Sims and Rotem down at a Hollywood Hills bash about 2.30am.

"He infiltrated the grounds and said, 'Are you Alli Sims? Here is a legal document ... you have been served,"' a source, reported by the New York Daily News, said.

"Cohen knows his stuff. He was in the Israeli Army."

Spears herself was reportedly inside at the party and was said to be furious.

Kaplan filed documents in an LA court last week demanding Federline get full custody of 11-month-old Jayden James and Sean Preston, 23 months old.

In recent weeks, Spears has been photographed out late with her boys, whereas previously she had been criticised for dangerously driving with Sean Preston in her lap. He also fell from a broken high chair when he was six-months-old and being cared for by a nanny.

She also took a tumble as she left the Ritz-Carlton Hotel in New York in May last year holding Sean Preston in one arm, and clutching a drink in the other hand.