Kevin Federline reflects on his past marriage to Britney Spears in his upcoming memoir, You Thought You Knew, and makes some wild claims about his ex. An excerpt from the book published via The New York Times on Tuesday, October 14, alleges that Federline, 47, and Spears' two sons - Sean Preston, now 20, and Jayden James, now 19 - were afraid of staying at their mom's house. "They would awaken sometimes at night to find her standing silently in the doorway, watching them sleep - 'Oh, you're awake?' - with a knife in her hand," Federline writes.