The topic below is a sensitive one. Just so you know in advance.
First, use swimsuits to teach private parts. It's up to you if you want to use anatomical names or not. Based on swimsuits, you can show that girls have two private areas and boys have one.
Second, was a little saying: Private parts are private. No one should touch them except for a few adults who need to help keep you clean and healthy.
Then define clean and healthy: wiping, washing, doctor check-ups, and in the case of my girls, applying ointment to yeast infections. Give examples of adults who might need to help keep a kid clean and healthy: caretakers and doctors. Every one's list of caretakers varies.
Teach your kids to say "No" and move away if they aren't sure a touch is appropriate. And teach them that any adult who asks them to keep a secret from Mommy and Daddy is deceitful and must be not be trusted. Always tell mommy and daddy. (Of course, Hadley wanted to know about surprise presents...there are always exceptions).
We can't guarantee safety for our kids since most of can't, and don't want to, be with our kids all the time. The stats about how most abuse occurs by someone a family knows and trusts is alarming. I think teaching our kids a few simple ideas could be helpful. Has anyone had similar conversations with their kids? Any suggestions?