Disick, 32, was photographed Wednesday on a father-son outing with Mason, 5, taking him for dinner in Calabasas, California, and buying toys at Barnes and Noble. On Keeping Up with the Kardashians, Disick has said the child he affectionately calls "Mase" is his "best friend."
Kardashian, 36, has also been spending time with their three kids: Mason, 3-year-old Penelope and 7-month-old baby Reign.
"Goodnight," she Tweeted on Wednesday along with a photo of children's bedtime stories.
Goodnight. pic.twitter.com/osmWi6dyXk— Kourtney Kardashian (@kourtneykardash) July 23, 2015
Though she's been putting on a brave face, a source who knows her says she's heartbroken by the split.
"Kourtney is miserable at the moment, so miserable," the source tells PEOPLE.
The trouble began when Disick – whose problems with substance abuse are well-documented on KUWTK – was photographed partying with another woman in Monte Carlo earlier this month.
Disick is scheduled to head to Las Vegas for a nightclub appearance Friday.