I believe in talking about sex with our children from the youngest of ages. It may start with simply naming body parts.
“Did you know a “pee pee” is actually called a vagina on a girl? Boys don’t have vaginas, they have a penis”.
I know for some parents just saying those anatomical names out loud is difficult, but it’s good practice, since the topic is only going to get more detailed with age. There are some great books that you can find in your local library that are written and illustrated specifically for preschoolers. Here are a few that I like. I am sure there are more. Please share you family’s favourites in the comment section and help other parents find good resources too.