The real question is, "Why is he peeing in the pool?"

Peanut: Mom will probably just tell us she's having another baby for her April Fool's joke.
Me: That's not even funny. Besides, we can't have any more babies.
Moon: Yeah. Remember? Dad had that ball surgery.
Me: What does a ball surgery have to do with having babies?
Moon: Remember, he just had a part taken out from his balls so now he can't have any more babies.
Me: Right. But do you know what balls have to do with having babies?
Moon: No idea.
Me: Do you want me to tell you about it?
Moon: I dunno.
Peanut: Yes, mom! How do you get babies?
Me: Moon, you really have no idea where babies come from?
Moon: No idea.
Peanut: Tell us, Mom!
Me: Have you ever heard of sex?
Peanut: No. What is it?
Moon, sheepish: Well, sort of.
Me: So, what do you think sex is?
Moon: I don't want to tell you.
Me: It's perfectly fine. We're having a conversation about it, there's nothing wrong with it, let's just talk.
Peanut: Mom, what is it?
Me: Moon, what do you think sex is?
Moon: Well, it's when a mom and a dad get naked and lay down in a bed together.
Me: And then?
Moon: I don't know.
Me: They're naked together and then the man puts his penis in the woman's vagina.
Peanut: I am NOT getting married!
Moon: Ew! Ew!
Me: It's not ew, and it's not only married people who have sex. Some people believe you shouldn't have sex until you're married, but I think it's okay when you are mature enough to handle it. Like when you go away to college and you're, I dunno, like 18, 19, 20 ish. In my opinion that's a good age to start having sex if you have a girlfriend and she is someone you love.
Moon: You still didn't explain about the balls.
Me: Oh, yeah. So, anyway, the woman has these little eggs inside her body, and once every 28 days she releases an egg and it's ready to be fertilized. So, in order to fertilize it, the man puts his penis in her vagina and this stuff called semen comes out of the hole in the tip of his penis.
Peanut: Really? The peehole?
Me: Yes. Same hole, different tube.
Peanut: Can I see a picture of semen?
Me, instantly terrified by the thought of Googling "semen" : No. It looks like snot, ok?
Boys in unison: Ew!
Me: So, this snotty stuff called semen is full of these tiny little things that look like tadpoles. They have long tails and they swim up the woman's reproductive tract looking for an egg to fertilize. If there's no egg, then no baby is made. But if there is an egg and a sperm swims into it, then a baby starts growing inside the mom's body.
Moon: That's disgusting.
Me: Yeah, well, guess what, buddy! There's more. When you get to be a little bit older, you might start having something called "nocturnal emissions," or "wet dreams." If you have too much semen in your testicles, it will need to come out, so sometimes it will come out while you're sleeping. That's why they call it a "wet dream." It doesn't mean you peed the bed. So, if this happens to you, you can just take the sheets off your bed and tell me they need to be washed. I won't ask you any questions, ok?
Moon: Ok. Hey mom, I had a wet dream once.
Me: What?
Moon, laughing: This one time, I had a dream that I was in a pool and I peed, and then I woke up and I had wet the bed.
Me: Not a wet dream. If there's no semen, you just peed the bed.
Peanut: Can we stop talking about this?
Me: Sure. Do you have any questions about anything else?
Peanut: Where does chocolate come from?