Ashlee Simpson and Pete Wentz rocked out and Jessica Simpson let her chest hang out at the Kenna concert on Monday night in Los Angeles. Ashlee and Pete, 27, arrived at hipster club Element with a posse of friends at around 9:30 p.m., where they said hello to Jessica Simpson’s former assistant Cacee Cobb, who was sitting two tables over with her boyfriend, Scrubs actor Donald Faison. Our Usmagazine.com night crawler tells us the group ordered a bottle of Johnny Walker Black Label, from which Ashlee, 22, poured herself drinks all night.According to our onlooker, the littlest Simpson “looked great” decked out in black skinny jeans and a matching top. “She had stars in her eyes as she sat on Pete’s lap for about 30 minutes before the show started,” the source tells Usmagazine.com. “She looked really happy.” Jessica, 26, arrived solo about 15 minutes after her sis, wearing a plunging black top, and joined the group. One partier described Jessica as “wearing a shirt down to there.” When Ethiopian-born rocker Kenna took the stage, the happy couple stood up on the booth and danced the entire time. Jessica (and her two “sisters”) stood up for the first two songs, then sat down by herself for the rest of the show. But she wasn’t ready to quit partying. She skipped out during Kenna’s encore performance and headed to L.A. hot spot, Parc, with several girls from Ashlee’s table.Source: USMagazine.com