Sara says unlike 6ix9ine -- who can afford hired muscle to keep him safe while he stokes feuds with other rappers -- she and their 5-year-old, Saraiyah, are much more exposed ...

and she's concerned they might become a target for his enemies.

6ix9ine's baby mama also claims he hasn't made any attempt to rekindle his relationship with Saraiyah -- so he's oblivious to the potential danger to her.