While Britney Spears schlumped her way through her VMA performance last night, Kevin Federline threw a backyard pool party for their two sons' birthdays. Personal assistants were seen picking up banners, balloons and an ice cream cake, all in the theme of the animated movie Cars, for Jayden James, who turns 1 on Wednesday, and Sean Preston, who turns 2 on Friday. Spears's parents, Jamie and Lynne, stopped by for the party at Federline's San Fernando Valley home, joining about 20 guests. They had also visited their grandchildren the day before, while their daughter was hitting up the club scene in Vegas before here widely panned VMA performance.