Inside Angelina Jolie and Gwen Stefani's Surprise Friendship

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.
PHOTOS: Kingston's super cute style
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 Anistonattended the couple's L.A. wedding in Sept. 2002. "They've run in the same social circle [for a long time]," adds a source.
PHOTOS: Shiloh's adorable fashion choices
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."
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."
PHOTOS: More pics of Vivienne and Knox
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.."
PHOTOS: Zuma's precious looks
"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 premiere 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!"