In my part of the world it's still relatively rare for the mother to voluntarily give up custody, and I do think many people (unfairly) judge women for making that choice. I can understand your concern and some readers may demonize your mother character no matter what, but writing against that outdated stereotype has got to be a good thing...right? I'm sure you'll feel your way towards the best way to handle writing those parts of your book. Good luck =D