How Much Do Kids Really Know

Here's the conversation we had tonight after reading this verse: "Joseph my last born"
Sam: What does last born mean
Me: Sam you know what last born means. The last child.
Conner: Lucy's the last born in our family.
Sam: No she's not. We're going to have more kids.
Me: (Suprised) We are?
Sam: Yes, we're going to have more.
Me: Are we going to have a boy or a girl or both?
Sam: Both but then he says a girl first
Lucy: Yay, Yay! (We are sitting in the living room and Lucy is upstairs supposedly asleep with her door closed. Haven't heard a sound from her for 2o minutes but as soon as Sam says a girl Lucy chimes in with a loud Yay, Yay! We were all laughing so hard. It makes me wonder how much do these kids really know. I don't feel like my family's complete yet but I have no clue when another addition will be in the works. Maybe I should ask Sam.