Pic: Kevin golfing with a pal in Los Angeles Tuesday
A source close to Federline told Us earlier this year that Prince is "so good" with the boys.
The couple -- who bowl together on a team named Party Animals -- "started off as friends, but it's become more," the pal added. "It's serious."
Unlike ex Britney Spears, the laid-back Prince "brings sanity and normalcy to his life," the friend adds. "He's a lot less closed-off now, so everybody is thankful he has her."