Pete Wentz and Ashlee Simpson-Wentz
09/06/2009 AT 12:00 PM EDT
, checking out hubby Pete Wentz
's band Fall Out Boy play a secret show at the Troubadour in West Hollywood. The new mom arrived with her parents – Joe and Tina Simpson – to see the group preview songs off their upcoming album, Folie à Deux
. But she didn't pull an all-nighter. "They're new parents," the source says. "They didn't want to be away from the baby too long."