Simpson appeared at the Orange County Fair in August to sing with Willie Nelson, her co-star in 2005's The Dukes of Hazzard.
"My good buddy Jessica Simpson and I are gonna sing a song together," Nelson told the crowd before Simpson joined him on stage to sing a new song, "I Will Be Your Fool."
For the performance, Simpson wore a loose black dress with silver polka dots and a black belt along with a pair of knee-high cowboy boots.
"It's not every night that a legend invites you to join him on his stage," she wrote on Instagram after the show. "And my husband and kids got to see me perform for the first time! I love you @willienelsonofficial."
She also shared a snap of her family backstage, with the singer and husband Eric Johnson posing with their children, daughter Maxwell and son Ace.
"Backstage shenanigans," the designer captioned the sweet moment.
Simpson revealed on Tuesday that she and Johnson will be adding a baby girl to their family, with Maxwell and Ace on hand to help their mom make her big announcement.
A pair of photos shared by Simpson see the brother and sister holding large black polka-dotted balloons before the balloons are popped to reveal smaller pink balloons inside.
"This little baby girl will make us a family of five," Simpson wrote. "We couldn't be happier to announce this precious blessing of life."
Later that day, the soon-to-be mom of three shared a snap of herself which showed off her growing baby bump.
"My Baby Love," she wrote.
In May of last year, Simpson told Ellen DeGeneres that she and Johnson weren't planning on expanding their family larger than two children.
"I have two beautiful children, and I'm not having a third," she said. "They're too cute … you can't top that."
"I'm not pregnant," the designer added. "We got an IUD, nothing's gonna get in that uterus."
Earlier this year, however, she admitted to Entertainment Tonight that she has "baby fever" from time to time, though she noted that she and Johnson "don't really know" if they want a third child.0comments
"We always practice," she joked. "But it would definitely have to be a miracle."
Photo Credit: Getty / Raymond Hall