In the photo, Carey, 49, and her eight-year-olds smile alongside Reynolds 43, and Lively, 32. Instead of smiling like his mother, however, Rocky donned a Deadpool mask and crossed his arms.
"Rocky snuck up on an unsuspecting couple wearing a Deadpool mask," Carey captioned the photo. "Will they ever recover? 😉💖"
Reynolds, best known for playing the superhero, would later comment on the post, saying: "Whoa. My vision board became a photo."
Just last month, Reynolds and Lively welcomed their third child together. The couple has yet to announce a name. The two also share daughters James, 4, and Inez, 3.
Carey shares her children with ex Nick Cannon.
Carey is currently working on a memoir, which will detail her meteoric rise to superstardom. The book is slated for a 2020 release.