The Blink-182 drummer, 45, shares stepdaughter Atiana, 22, son Landon, 17, and daughter Alabama, 15, with ex-wife Shanna Moakler, whom he was married to from 2004 to 2008. All three of the children posted on Instagram following the Sunday evening engagement.
"Congratulations @kourtneykardash and @travisbarker I'm so happy for you guys love you guys so much!" Landon wrote in an Instagram Story over a photo of the newly engaged pair kissing.
"So happy for you guys I love u both!" Alabama echoed on Instagram while giving a look at the sparkly engagement ring on Kardashian's finger.
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Atiana also posted from the special night, sharing a pic of Kardashian's new rock as well as an image of the reality TV star and Barker sweetly kissing.
Landon Barker Instagram
Kardashian, 42, and Barker got engaged in a romantic, beachside proposal on Sunday at the Rosewood Miramar in Montecito, California. He popped the question at sunset surrounded by candles and roses in the shape of a heart.
Axelle/Bauer-Griffin/FilmMagic; Inset: Frederick M. Brown/Getty Images
The Keeping Up with the Kardashians star confirmed the exciting news on Instagram by sharing pictures from the romantic proposal with the caption, "forever @travisbarker."
"It was a complete surprise for Kourtney," a family friend told PEOPLE. "Travis told her family that he planned to propose, but everyone was great at keeping it secret. The Rosewood is their favorite for short weekend getaways. They often stay at the hotel. This is why Travis proposed there. It's a very special place to them, but it was also easier to keep everything a secret by proposing there."
Alabama Luella Barker Instagram Kourtney Kardashian and Travis Barker
"Kourtney is of course ecstatic. She can't wait to marry Travis. She would also love to have a baby with him," the source added.
RELATED GALLERY: Kourtney Kardashian and Travis Barker's Sweetest Photos Together
This marks Kardashian's first engagement. She shares her three children — Mason, 11, Penelope, 9, and Reign, 6 — with ex Scott Disick. In addition to Moakler, Barker was previously married to Melissa Kennedy from 2001 to 2002.
Atiana De La Hoya Instagram
PEOPLE first confirmed Kardashian and Barker's relationship in January. They made their romance Instagram official in February after Valentine's Day, when the reality star shared a photo of the pair holding hands.
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Shortly after they started dating, a source said Barker had gotten the seal of approval from Kardashian's family.
"Her family and friends all really like him," the source said. "Their kids all get along too, which is sweet."