Generally children under 9-years-old ask questions about (1) things they have heard and seen grown-ups talking about and doing and (2) things their friends have heard and seen grown-ups talking about and doing. So the long and short of it, if the child is asking, as the adult you have to answer because the kid has some personal knowledge they’re trying to work through. But before you jump headlong into an elaborate, wordy explanation of oral sex and mutual pleasure, stop and take a breather. Ask the kiddo some questions to get a clear picture of just exactly what they are asking. It’s probably nothing like what you assumed.
However, if your kid really is asking what you were worried they were asking, you can feel free to say: A man’s sperm meets up with a woman’s egg to make a baby, and then the baby grows in the woman’s tummy. If questioned further, you can even say that the sperm and the egg meet in the woman’s tummy because that’s where the baby grows. Further question can be followed up by saying that the man’s sperm gets to the woman’s tummy by the man’s penis going inside a woman’s vagina and leaving some sperm inside her. That will probably be enough to seriously gross most kids out. Follow that description up by saying that’s an adult thing to do, kind of like driving, voting, and drinking alcohol.
Particularly in this age of visual images, so many of them sexual, bombarding our lives, it is important to address what your kids are seeing around them. Teach them to analyze and criticize hyper-sexual images. Feel free to talk about why it’s inappropriate for children to wear make-up and bikinis, why it’s silly to sell widgets by putting almost-naked women (and occasionally men) on top of said widget, and how girls and boys bodies are different and how they evolve into the differences in men’s and women’s bodies. Feel free to talk about how it’s silly that some people think marriage is only for one man and one woman and that it would make more sense for any two people to be able to be married, whether they’re boys or girls. (Most kids have an innate agreement with this point, because they’re generally focused on same-gender friendships at that age.)
But before I leave off this topic for the moment, I want to impress on you the lightness of most children. They, by and large, haven’t been exposed to much sexuality in other people. They may or may not be enjoying their own sexuality yet. So before they’re 9 or so, they just don’t need much in the way of explanation – just what it is, and whether it’s okay or not. It’s that 9-and-up set that start needing and wanting more explanation and conversation.