Personally I would have just gone for the light science approach.
"It'll all sound pretty crazy but nature is complicated and weird, so it's pretty normal as you learn more about the world. All the instructions for making a living thing are in a chemical called DNA that's inside every single living thing. Your hair, the colour of your skin, what your face will look like, the colour of your eyes, they're all decided by what your DNA says to do. So when people want to have a baby, they have to put their own unique DNA together, to create a new set of instructions. Half of your DNA comes from your mom, and half of your DNA comes from your dad, and that's why you're a little bit like both of them. Understand?
Many living things put their DNA together by having sex. For humans, that means that a man's penis gets hard and grows bigger, and he puts it inside a woman's vagina. His DNA crosses into the woman's body in white goo called sperm that comes out of his penis. The woman has a tiny cell living inside her called an Egg, which has her own DNA inside it. When sperm finds the egg, they combine their DNA and create a new set of instructions for making a new human being. That eggs starts growing inside the woman's body, and becomes a baby over the next nine months by following the new instructions. Understand that?
There are lots of ways for living things to reproduce themselves, some animals don't have sex at all, and plants get help from bugs like bees, or even the wind to carry their DNA around to other plants. Some animals like humans and dolphins even have sex for fun, even if they're not trying to have a baby. And sometimes people have trouble getting their sperm and egg to work together, so they get help from doctors to put them together. Understand that?
As weird as it all sounds, whether you lay an egg when you're a bird or a reptile or a fish or an insect, or you grow the baby inside the female's body and it comes out of her vagina like what happens for a human, a whale, a dog or a lion, living things always make new living things by following the instructions in the DNA. And sex is how humans combine their DNA."
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