I read something once that children who know the proper anatomical names for their genitals are less likely to be abused, and more likely to speak out if they are abused.
It's because they're less likely to feel ashamed about talking about it. If mum and dad can't talk about genitalia using proper terminology, how can they? So the affected kids take longer to figure out how to say something.
And as CBG says, it stands up in court if the awful does happen.

Definitely recommend using penis and vagina.