Garner, who was married to the actor from 2005-2018, "can tell he is happy," the source says, adding that Affleck and Lopez's families are "getting along great." Affleck shares three children -- Violet, 15, Seraphina, 12, and Samuel, 9 -- with Garner, while Lopez shares 13-year-old twins, Emme and Maximilian, with ex-husband Marc Anthony.
"J.Lo and Ben's families are getting along great and they are making an effort to take things slow when it comes to their kids," ET's source says. "[J.Lo] is so happy with Ben. He is so laid-back and chill and he makes her laugh nonstop. He is caring, sweet and supportive of her, but not competitive with her when it comes to their careers and fame."
"Ben just gets her and it's nice and fun when they're together," adds the source. "They have so much in common and it's like they never ended their relationship years ago. They have picked up right where they left off."
As ET previously reported, Lopez and Affleck, who fans originally dubbed Bennifer, were previously engaged from 2002 to 2004. The "Let's Get Loud" singer reconnected with Affleck earlier this year, shortly after ending her engagement to Alex Rodriguez.
"ARod has been working on himself and taking care of himself," a second source tells ET. "He regrets some of his actions and feels like he messed things up with Jen, but he is doing his best to move on."