Robin Thicke -- Merry Ex-Mas WITH Paula Patton

Robin Thicke and Paula Patton spent Christmas Eve partying together -- and even though they're not reuniting -- based on pics from the bash ... some of her family wishes they would.

Robin looked like the most popular guy at the holiday fest thrown at Paula's house -- and her cousin posted photos of the singer all buddy-buddy with everyone in the fam. Except Paula.

A source close to Robin tells us it was totally normal for Paula to invite Robin because of their kid, Julian. But c'mon -- look at the photos -- it's not normal for her family to show him so much love.