M.L.
She may get frightened some more. She'll wonder what will happen and will take the initiative in defending herself. Some cats are also divas, worrying about their status at the drop of a paw.
One of my grandcats is like that - not cuddly, not a very, um, gracious vocabulary, pretty worried and defensive all the time. Of course, that's understandable since she spent her early weeks in a college dorm, poor thing. My granddaughters have been taught to let her alone, speak to her gently, and know that she's just not a love bug.
Consider giving the kitty some good places to hide. That might help, if she has her own territory. I assume you've taught your boy to let her alone unless she decides she wants to play. You can play with your kitty and stroke her when you have the opportunity, so she'll know you still love her. Talk to her gently, especially when she's in the same room as the baby (you might close off the baby's room to the kitties when you're not in it, for a while).
You might even get some tuna or cat treats that she loves, and give her some *only* when she's around the new arrival. She might connect the baby with something good that way. As a matter of fact, would a few treats make her more amenable to your son?
I guess what I'm saying is that you'll need to ensure the safety of both cat and baby until the cat decides the newcomer is acceptable. (It's HER house, after all!)