The singer and actress posed on the red carpet at the El Capitan Theatre on 5 August alongside blond-haired Bronx and his young friend.
Ashlee kept her rock chick edge in a black lace dress and black ankle boots, teamed with thick bouncy blonde hair and sexy smoky eye make-up.
Four-year-old Bronx looked every inch the rock child in his skinny grey jeans and vintage Star Wars T-shirt. His dad is Ashlee's ex-husband, Fall Out Boy star Pete Wentz.
Bronx recently became a cousin for the second time after Ashlee's sister Jessica welcomed son Ace. Jessica is already mum to daughter Maxwell.
"Right now, he's like 'OK, Maxwell, come hang out with me! Let's go to your play room,' " she told People. "We are always all together. We're a close family."
Absent from yesterday's family day out was Ashlee's new boyfriend Evan Ross, the son of legenedary singer Diana Ross.
Ashlee was recently introduced to Evan's mum at an after-party for her Los Angeles concert, where a source claims Ashlee and Evan were "very affectionate" with each other all night.