I'm not shy about talking to my kids about personal body stuff. I have the benefit of experience with my older kids and I used to work with the NHS on sexual health, so I got over any embarrassment long ago. It's far easier to talk to your children when they are young, and when they ask, but that doesn't always make it simple.
What's Down There? and
That's My Willy are newly released books from author Alex Waldron which are very honest, very clear and very blunt, and the messages they pass on are all positive and essential messages that I have told my own children at some point.
Both books follow a similar light-hearted rhyming format, first introducing the main character, and then their genitals. It is made clear that these are your own. They're your private parts and it's fine to touch them and explore your own body. It's also fine to say "No" to other people if you don't want them to see or touch you.