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 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.

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."

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.

PHOTOS: Zuma's precious looks

"All of the kids are very close in age. They've attended each others 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!"

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