I don't think Rashta would kill her first kid. Her reactions in recent chapters makes it feel pretty clear she cares a little TOO much about them, I think it was stated the main reason she ran away was because she was told the baby died and the emotional toll of having a child while a slave was just too much. She's flawed and does plenty of bad stuff, but there's a HUGE leap from her current behavior to killing your own child.
But honestly, the easiest solution would be to kill the Viscount. Then she'll have *nothing* to fear, except the child's resemblance to her.