Nicole Richie and Joel Madden's son Sparrow Midnight James is growing up fast!

The only son of the celeb couple celebrated his 4th birthday in Beverley Hills on Saturday, Sept. 6 with a Spiderman superhero-themed party, which boasted a star-studded guest list—and In Touch is hearing all about it!

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"It was a very posh event for a kids 4th birthday party, and of course they had valet,” an eyewitness told In Touch. “There were Spiderman banners and balloons draped all over the backyard. Dad Joel even got into the spirit and wore a Batman tank top in honor of his son's bday."

Some of the attendees included a pregnant Gwen Stefani accompanied by youngest son Zuma, wearing a houndstooth onesie to cover up her baby bump.

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"She tried to keep her tummy covered with her big white bag in front of photographers, but she's definitely starting to show," the eyewitness added. 

The original "Spiderman," Tobey Maguire, didn't make it, but wife Jennifer Fisher brought their kids Ruby and Otis, who were wearing matching stripes, according to an eyewitness. 

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Cash Warren, whose hair is starting to turn completely grey, also came with his daughter Honor and Kourtney Kardashian made a quick appearance to drop off a gift, sans kids, and kept her sunglasses on. 

"The party was definitely a who's who of the Hollywood kids set," adds the eyewitness. "These kids might be the ones running Hollywood someday!"

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