Randall,
There's a strong chance this woman is a cluster B woman (BPD, NPD, HPD), and even she isn't, her behavior is toxic.
This is an extremely dangerous situation for not only you but also your child.
You cannot assume that normal social behaviors or morals apply to this woman. She can and will use her child as if the child is a bargaining chip.
Anyone who has been in this situation will tell you that one of the issues was that they couldn't believe their girlfriend would do something so disrespectful, disgusting, outrageous until it was too late and they saw it with their own eyes.
Nothing is off the table every minute you stay the worse the potential outcome becomes.
It is not your job to rescue her child. Is it sad? Absolutely, but your girlfriend is exploiting your bond with her child to keep you enmeshed in the relationship.
Ask anyone who has been in these relationships what the fallout is and lucky ones that get out have a few years of PTSD, the unlucky ones end up with no money, a complete loss of self, and/or dead.