Ben Affleck Seen Having His Kids Hang Out with One of Jennifer Lopez's

Ben Affleck was on dad (and possibly stepdad) duty this weekend -- with not just his kids in tow, but with one of the children of his beloved, Jennifer Lopez, too ... the first time we've actually seen that happen.

The actor was out Sunday with his son, Samuel, his daughter, Seraphina ... plus, J Lo's 13-year-old daughter, Emme -- who was out there rockin' the blue-green hair. Seraphina is right around Emme's age ... and Sam's a bit younger at 9. Still though, they all seemed chummy.

Ben, meanwhile, had his hands full ... with a ton of Wetzel Pretzel snacks and some drinks for good measure. They were out at the Universal City Walk, with some adult companions (possibly nannies/security guards) tagging along as well. No sign of Jen or her boy, Max, here.

No matter ... because this itself is a HUGE step toward blending their family units -- much like Jennifer was doing with A-Rod when they were engaged not too long ago. Their kids were constantly hanging out with one another -- and now, it seems Bennifer's doing the same.

There's even more evidence that Jen and Ben are seriously intertwining their personal lives -- just a couple days ago, J Lo was spotted out with Max ... touring a school in L.A. We'd already seen her doing some of that on her own a little while ago -- but this time, she brought the kid to check things out too.