Robin Thicke's 40th Birthday Bash Totally Kid-Friendly

Robin Thicke made sure his 40th birthday was literally fun for the whole family -- adults and his 6-year-old son alike.

Robin hosted a luau-themed bash at his Malibu pad Sunday, and Julian was in the thick of it with some cousins and friends there too.

The adult guests included Adrien Brody, Angelica Bridges, Russell Simmons, Orlando Bloom and his son ... and, of course, Robin's gf April Love Geary.

As for the party -- magic show, 2-story water slide, mechanical shark, petting zoo, face painting ... in other words every kid's dream. They had hula dancers too ... fun for kids and adults.

Sources close to Robin tell us he was thinking "family first" when he planned the event.

Paula Patton was not there, but as we reported ... she and Robin are getting close to settling their custody war.