The future college student was seen out and about in New York City on Wednesday with her grandmother, Bonnie Haydon, and her aunt, Veronika Haydon.
Kai looked casual in blue jeans and a black sleeveless top as she sat down for dinner with them at The Mark mere hours after Vanessa Trump announced the news.
The mother of five took to Instagram to reveal her diagnosis. "While this isn't news anyone expects, I'm working closely with my medical team on a treatment plan," she wrote.
"I would like to thank my doctors for performing a procedure earlier this week on me," Vanessa added, before sharing that she was "staying focused and hopeful while surrounded by the love and support of my family, my kids, and those closest to me."
"Thank you for your kindness and support, it truly means more than I can express. I kindly ask for privacy as I focus on my health and recovery."
Kai showed her support by posting a photo of her mother by her side, with the caption: "The strongest person I know."
Vanessa's post comes just days after Kai graduated from The Benjamin School in Florida, following a ceremony attended by her mother, her father, Donald Trump Jr., and her mother's partner, Tiger Woods.
Kai's friend shared a series of snaps from the special day that saw the YouTuber in a white sundress with a navy blue graduation gown over the top and a cap on her head.
Hours later, Kai was pictured attending a celebratory dinner in Palm Beach, alongside her aunt and uncle, Tiffany Trump and Michael Boulos, and her father's fiancée, Bettina Anderson.
While her other aunt, Ivanka Trump, was not in attendance, the 44-year-old did share her support for Vanessa in the wake of her breast cancer diagnosis. "Praying for your continued strength and a swift recovery. Love you mama," she wrote on Instagram.
Vanessa and Ivanka are incredibly close despite the former's split from Donald Jr. in 2018, with Ivanka calling her a "wonder woman".
"My sister-in-law Vanessa is a machine," the mother of three told People. "She can take all of [my children] and not even notice. She has five children, and she's Wonder Woman to me."
"She can be taking care of them all simultaneously, each pulling on her and have direct meaningful connections with each of them at one time. It's remarkable."
Vanessa and Kai's close bond was evident in a touching post that the blonde beauty made in honor of her daughter's 19th birthday in May.
"I am beyond proud of you and all that you have accomplished. Watching you graduate from high school and begin this new chapter into adulthood is one of the proudest moments of my life," she wrote on social media.
"You have worked so hard to get where you are today, and it truly shows. You have grown into such an amazing young woman, smart, strong, kind, and determined."
She continued: "From excelling in school to being an incredible role model for your siblings and everyone around you, you continue to inspire me every single day. I love you with all my heart, and today is all about celebrating you."