Machine Gun Kelly is firing back at his ex's ex Brian Austin Green by honoring him with an exclusive drink he's claiming will wash away any potential issues with adulthood -- and help him mind his own beeswax!!!

On Friday, MGK's 27 Club Coffee introduced the "Child Actor Margarita" today, an ALL-GREEN drink intended to "put a childish twist ... for those days when you just don't wanna act like an adult."

To add a little salt into the wound -- and margarita, the coffee club added a "9021-OMG" to the caption.

Sources close to MGK say it's just a light-hearted jab, considering that B.A.G. has been throwing heavy shots at his maligned relationship with Megan Fox.

We caught up with Brian shortly before MGK and Megan's baby girl was born ... and he didn't mince his words urging Kellz to grow up.

The drink is currently being served at MGK's Cleveland Club Coffee location ... just in case anyone else needs to drown out those childish blues.