It was a double-date night for new dad Joel Madden
as he and brother Benji celebrated their first DCMA Collective store in Los Angeles with girlfriends Nicole Richie
and Paris Hilton
"It's like another baby!" Joel told PEOPLE of the eight-months-in-the-making store off L.A.'s famous Melrose Avenue. "We're going to carry DCMA's line [including] baby stuff, the jewelry – it's my favorite – and the collaboration T-shirts." Adds Benji: "We come from a small town, and not very much, and to be doing this – it's definitely a dream come true."
Richie and Hilton helped their significant others entertain guests including Pete Wentz, Nicky Hilton and David Katzenberg, the Hills'
Audrina Patridge and Christina Milian, who also spent time at sponsor RCause.org
's booth, a new organization that supports charities including the Richie-Madden Children's Foundation.
"It's so cool to see this come to life because this has been Joel's dream for a very long time," Richie, in a vintage Jil Stuart gown, tells PEOPLE. "When I was pregnant I would always wear Joel's DCMA T-shirts, but now I'm excited to wear the girls' stuff. And Harlow wears custom DCMA."
So where was 9-week-old Harlow on her dad's big night? "My mom's with her," said Richie. "It's fun, but it's bittersweet because I don't like to leave her. And I always wonder, 'What is she doing?' [I've checked in] twice so far – and I've only been gone an hour! But this is my adult night so I'm excited to see what the night brings."
Although the couple ended up heading home instead of joining Benji, the Hilton sisters, Katzenberg and Patridge at a private afterparty at Hollywood club Central, nursing mom Richie was prepared in case the night stretched on. "[My breast pump] is in the car. And we have a converter because there's no car charger, so that way I can plug it right in at 10 o'clock!" she said.
Next up for the Richie-Madden clan: saying goodbye as Good Charlotte kicks off a four-week South African and European tour next week. Joel says he plans to stay in touch with his family via online video links so he doesn't miss a milestone.
"It's all about iChat," he tells PEOPLE. "[Harlow] is giggling and laughing all the time now. She wakes up laughing every day – we're really blessed."
With reporting by REAGAN ALEXANDER and ALONDRA HERNANDEZ