Ben Affleck and Jennifer Garner touched down at Van Nuys airport Thursday after a family vacation TOGETHER in the Caribbean for the kids' spring break ... yes, TOGETHER!

These two are very successfully co-parenting their three kids -- Seraphina, Violet, and Samuel -- and taking the somewhat unusual step of continuing to do family vacations together for the sake of the kids. #Mature!

A source tells X17online the group was in Parrot Cay, Turks and Caicos -- where Ben and Jen were MARRIED in 2005!!! Seems kinda crazy they went back to the site of their nuptials, more than a decade after Ben put a ring on it, their marriage has dissolved. But, no harm, no foul. It's a tough rode, but they're keepin' it together!