Cardi B and Offset put on a friendly front at their son Wave's birthday bash this weekend ... but TMZ has learned the rappers are still very much broken up.

Sources with direct knowledge tell TMZ ... Cardi and Offset's divorce is still on and moving forward, despite their recent reunion at DreamWorks Water Park inside American Dream in East Rutherford, NJ.

We're told the divorce remains an amicable one ... which is why they're OK being in the same space while celebrating their children -- including daughter Kulture, son Wave, and an unborn little one on the way.

Sources say there has never been a discussion about not being around the kids at the same time ... and that's thanks to their friendly co-parenting dynamic, on full display during Wave's 3rd birthday party.

We're told there's no bitterness or strife as the duo looks to legally end their marriage.

Cardi B filed for divorce -- for a 2nd time -- from Offset earlier this summer ... after the twosome had been on and off for the last 4 years. At the time of Cardi's filing, sources told us the divorce was "a long time coming," since she and Offset had grown apart.

Cardi later confirmed her 3rd pregnancy, in which she hinted at the end of her marriage.

She wrote ... "With every ending comes a new beginning! I am so grateful to have shared this season with you, you have brought me more love, more life and most of all renewed my power! Reminded me that I can have it all!"

Cardi proved there was no bad blood between her and Offset ... she came to her baby daddy's defense when he was accused of not supporting her during their relationship.

It's clear these 2 are still family ... no matter the status of their relationship.