I'm going to tell my FWB/baby mama that she doesn't just "deserve" a child
Why do you think that she does not deserve a child?
I'm not sure if I want to have a kid with her.
It's good to set boundaries with a sexual partner. This way you both know where you stand in the relationship and what to expect. She might be upset to hear this but this allows her to make the choice she feels is right for her.
I'm going to continue my plan to tell her that this is her last chance, I can't believe my baby mama thinks she deserves a child.
You should talk to her in a nice way about the fact that you do not want to have a child. You should not tell her that she deserves not to have a kid though. There are better ways to go about it.