Talking About "IT" with Kids (all ages)
Whether you really can't bring yourself to say the word "vulva" out loud, or like me, say all those words all the time even to my children, I think we can all agree that we would rather be the gateway of information about sex to our children rather than the internet. The internet is full of amazing, medically accurate, age-appropriate information, but it is harder to find than free porn! And where do we want our children getting their notions about sex?
Be Trustworthy! You can not under any circumstances lie to your children about sex. They will all too soon discover your lie or even mischaracterization and instantly you will no longer be a trusted source of information. If it's not something you can answer, use phrases like, "I'm not comfortable talking to you about that right now, but since you're asking important questions, how about we get you a book you can read that will answer some of what you're asking." Then you are still the gate keeper. You will not be the gate keeper forever, but while you can make a difference, make sure you're a person they can trust about this.
Younger is Better Things you should never say: "You're not old enough" or "I'll tell you when you're older." Kids resent that. Remember back when you were a kid, wasn't that the most frustrating thing? And really earlier is better. Having basic facts their back pocket when they inevitably hear false or exaggerated information, or heaven forbid they get sent a porn GIF over social media, will give them confidence to refute or to ignore. If they have real information, from a trusted person, with which to compare and contrast, they won't be shocked, they won't be overwhelmed, and they won't see the new information as automatically true. Teach your children the proper anatomical names of their body parts before Kindergarten (this also helps prevent child abuse), that sex is the term for human fertilization during elementary, what porn is during junior high, and what sex is and the pressures that will arise around the subject before they can drive.
Teach Active Consent You can teach consent in every day activities, from as early as 2 years old. Now obviously one doesn't talk about sex at 2, but it looks like: "He is saying stop, that means you have to stop. To keep grabbing his toy means you don't respect him, that he doesn't matter, and that's not how good friends act." This idea, used consistently through childhood, from preschool to high school reinforces respect of others and their wishes. Later it can look like: "Sure your sister was enjoying splashing each other in the pool, but now she's done, and you have to respect that and stop. I know it was super fun and you were both laughing a second ago, but once she's done with the game then the game has to stop." The idea of active consent is just there, a part of interacting with other people, in all situations. This also matters in parental behavior, if you or another adult keeps tickling a child past when they've said to stop, then you are teaching them that bigger and/or older people in their lives don't have to follow the same rules, and that their "no" didn't count because of a power differential. This is obviously a bad lesson to say the least, and one that also cracks the door to child abusers. Teaching kids to pay attention to their playmates and themselves, to tune into verbal and non-verbal cues, and to keep checking in to make sure everyone is having fun is how to be a good friend and community member, and later a good partner.
No Shame Teaching that their bodies are special and their own, without shame can also be a wonderful gift to give your child(ren). Every kid will be curious about their body, and the more normal, the most like answering questions about what their nose is for you can manage, the better. Now penises are different than noses, but if your attitude is similar it will help, and if there comes a time when someone makes them feel uncomfortable, they will be more likely to tell you if it's not such a big deal. They will ask awkward questions and they will touch themselves, and neither activity should they be made to feel shame about. That doesn't mean you don't tell them when those things are appropriate though. And if you can't be relaxed about any of this, there are lots of cute anatomy books for kids out there, ones with cartoon pictures that you could just leave around the house, and later on something like "The Care and Keeping of You" could magically arrive in the home library and later than that a book like "Dating and Sex: a guide for the 21st century teen boy" could be left on a dresser.