Over the Memorial Day weekend, the star and the Knives Out actress spent the night together with his three kids — daughters Violet, 14 and Seraphina, 11, and 8-year-old son Sam — at his home.
The source says Affleck, 47, is "very serious" about 32-year-old girlfriend de Armas. "He wants the kids to spend time with her so they can get to know her," the source adds.
On Saturday, the couple was spotted taking a stroll in Los Angeles with his kids, who he shares with ex-wife Jennifer Garner.
The group all wore face masks as his daughter Violet was seen walking with their family dog Birdie, who is owned by Garner.
"They are very happy together," a source told PEOPLE of Affleck and de Armas, who made their relationship public in March after meeting on the set of their upcoming movie Deep Water.
BG004/Bauer-Griffin/GC Images Ben Affleck, Ana de Armas
While the Saturday outing marked the first time de Armas was pictured with all of Affleck's kids, the source said the actor "continues to work on co-parenting" with Garner, whom he split from in 2015 and finalized their divorce in October 2018.
"Ben and Jen work together at that," the source adds of the exes who are sharing custody of their children.
Earlier this year, Affleck praised Garner for her parenting, telling PEOPLE in February that he felt "lucky" to have her as the mother of his children.
"When you have children with somebody you're connected to them forever," the actor said. "And I’m very lucky she is the mother of my children."
Affleck added, "I'm very grateful and respectful of her. Our marriage didn’t work, and that’s difficult. Both of us really believe that it’s important for kids to see their parents respect one another and get along, whether they’re together or not."
The Gone Girl star and de Armas have been social distancing together, and have been spotted on almost daily walks around his neighborhood in Los Angeles.