Meanwhile, Travis' two children, 17-year-old Landon and 15-year-old Alabama -- whom he shares with his ex-wife, Shanna Moakler -- took to their Instagram stories to share photos from the engagement celebration, and messages of love and support.
"Congratulations @kourtneykardash and @travisbarker I'm so happy for you guys love you guys so much!" Landon wrote over a video of the pair exchanging a kiss at the table.
Alabama, meanwhile, shared a photo of her dad giving Kourtney a sweet peck on the cheek.
"Kourtney and Travis are over the moon. Getting engaged was always their natural next step and they are so thrilled," the source said. "The proposal was gorgeous and romantic, and everything Kourtney wanted."
"Their families are beyond happy and excited for them too. Their kids are very much looking forward to being an official big happy family," the source added. "Kourtney and Travis really feel like they've found their perfect partner and match. They can't wait for this next step in their journey as a couple and to spend the rest of their lives together."
Kourtney, 42, and Travis, 45, have been longtime friends and neighbors, and started dating in January. The two quickly got serious, taking family trips with their respective kids. Kourtney shares three children -- 11-year-old Mason, 8-year-old Penelope and 6-year-old Reign -- with her ex, Scott Disick.