“They are serious,” the insider adds of the couple, who first connected in 2018

Jennifer Garner and John Miller . Photo: Kevin Mazur/Getty; Maciel / BACKGRID

Jennifer Garner is enjoying her “fun” relationship with John Miller while continuing to co-parent with her ex-husband Ben Affleck amid his recent split from Jennifer Lopez.

A source tells PEOPLE that Garner, 52, spends as much time as possible with Miller when she’s not with her three kids — Violet, 18, Seraphina, 15, and Samuel, 12 — whom she shares with Affleck.

“She loves [Miller],” a source tells PEOPLE, noting that the pair spent the weekend together as Affleck is caring for the kids.

“They are serious. He’s also a co-parent and understands her priority,” the source says.

 “Their relationship is fun and easy-going,” the source adds. “She’s very happy.”

Jennifer Garner.Getty Images

Meanwhile, on Saturday, Sept. 14, Lopez, 55, and Affleck, 52, reunited and went to lunch together with their kids at the Polo Lounge at the Beverly Hills Hotel in Los Angeles, nearly a month after Lopez filed for divorce from the actor, a source told PEOPLE.

Seraphina and Samuel, as well as Lopez’s 16-year-old twins, Emme and Max — whom she shares with ex-husband Marc Anthony — were part of the afternoon outing.

“They all came together for a fun lunch so the kids could hang out,” the source said. “[Jennifer’s] trying to be friendly with Ben. They are still moving forward with the divorce though. They are working out financial details amicably.”

Ben Affleck and Jennifer Lopez.Lionel Hahn/WireImage

The source added, “A divorce is never easy, but Jennifer doesn’t want to be selfish about it. The kids always got along and have fun together. It makes her happy seeing the kids happy together. Happy kids are her priority.”

Never miss a story — sign up for PEOPLE’s free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories.

Garner and businessman Miller first connected in 2018, shortly after the 13 Going on 30 actress finalized her divorce from Affleck, whom she married in June 2005. Though the pair were seen together over the years, Garner said she was single in a March 2021 interview with PEOPLE.

In February 2023, a source told PEOPLE that Garner and Miller “took a long break from dating, but Jen is very happy that they are back together.”

The couple were recently spotted on a date night in August at a Los Angeles restaurant. The two were photographed with smiles on their faces and dressed casually, with the actress in a blue sweater and Miller in a flannel button-up shirt over a white tee.