09/10/2007 at 04:00 PM EDT
While Britney Spears
struggled at the Video Music Awards in Las Vegas, her ex-husband Kevin Federline
threw a backyard pool party Sunday – for their two sons' birthdays, PEOPLE has learned.
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 two 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 the club scene
in Vegas before her widely panned
Spears, 25, and Federline, 29, are embroiled in an increasingly fierce custody battle
over their boys. They currently have 50-50 custody, but Federline is seeking more time, serving subpoenas on people in Spears's inner-circle to show problems in her parenting skills.
Reporting by PERNILLA CEDENHEIM