They were accompanied by their 2-month-old son, Charlie West, and Arquette's 10-year-old daughter with Courteney Cox, Coco.
The proposal occurred just days after word broke of Cox's engagement to Snow Patrol rocker Johnny McDaid.
"I'm engaged to him!" Cox wrote on Twitter, along with a photo of herself and McDaid.
Arquette married Cox in 1999 and separated nearly 11 years later in 2010. Their divorce was finalized in 2013.