The Jolie-Pitts and Stefani-Rossdales are BFFs! 

On Monday, Angelina Jolie and part of her brood went to Gwen Stefani's London home for a playdate. Shiloh, 5, Zahara, 6, and 3-year-old twins Vivienne and Knox were all smiles alongside Kingston Rossdale, 5.

So how exactly do Jolie, 36, and Stefani, 41, know each other? A source tells Us Weekly that Brad Pitt has been pals with the No Doubt singer and hubby Gavin Rossdale for years. He and ex-wife Jennifer Aniston attended the couple's L.A. wedding in Sept. 2002. "They've run in the same social circle [for a long time]," adds a source.

When Shiloh and Kingston were born a day apart, the parents became "even better friends. "Right away they started getting [the kids] together for play dates," a source tells Us. "Brad had [the Stefani-Rossdales] up to his Santa Barbara house for the weekend." See more pics of the star families' playdate.

In an interview with CNN's Sanjay Gupta last year, Jolie admitted she didn't have a lot of close friends. "I'll talk to my family," she said. "I talk to Brad [Pitt]... But I don't know. I don't have a lot of friends I talk to. He is really the only person I talk to."

Jolie and Pitt, 47, also welcomed Vivienne and Knox in Aug. 2008, just one month before Stefani and Rossdale, 45, had their son Zuma.

"Somehow [Gwen and I] keep ending up pregnant at the exact same time," Jolie told the Sydney Morning Herald in 2008. "We were having a playdate and Gwen was trying to figure out what she was going to wear as she got more pregnant... I asked her if she had any spare clothes, and she gave me this [L.A.M.B. casual black] dress.."

"All of the kids are very close in age. They've attended each other's birthday parties and gotten together several times at each others homes," says a source.

Over the summer, Stefani and Rossdale attended the premier of Pitt's film Tree of Life at Cannes, and now that both families are in London, a source says "it made perfect sense for all of the kids to get together for a playdate!"