The stars came together to celebrate youngest child Sam's 4th birthday party on Saturday at their L.A. home, a source tells PEOPLE.
"It was a morning party," the source says. "The kids had fun in the yard. Jen and Ben were both at the party. There was pizza and lots of snacks."
While Sam played with friends at the superhero-themed bash, guests also included Reese Witherspoon and Garner's Alias costar Victor Garber stopped by, according to the source.
Garner and Affleck are also parents to Violet, 10, and Seraphina, 7.
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Later on Saturday, Affleck and Garner were spotted taking their children out to dinner at Art's Table in Santa Monica.
"Jen and Ben seemed fine, you would never think that they are divorcing," a source at the restaurant tells PEOPLE. "They had a fun dinner with their kids and were both laughing a lot. The kids were very charming. Their son acted like a little clown and seems like an actor for sure. Both Jen and Ben were smiling as he goofed around. Ben had a burger and the kids had fries."
Another source, who spotted the family on their way to the restaurant, tells PEOPLE, "Ben and Jen seemed very happy. They were laughing in the car on the way to the restaurant."
The reunion comes shortly after Garner's illuminating and candid Vanity Fair interview – in which she called Affleck the "love of my life" – and just ahead of the Oscars, where the actress is set to present.
Affleck spent the night before Sam's big day at a William Morris Endeavor pre-Oscars party in Beverly Hills, California. The star's next movie (and first turn as the Caped Crusader), Batman v Superman: Dawn of Justice, is due out later next month.
After the party, Garner spent part of Saturday at the Dolby Theatre, learning stage blocking alongside other Oscars presenters like Olivia Wilde and Tina Fey.