Hmmm this is tricky! I'm trying to think of what my parents told me (I asked how a baby gets out mummy's belly button when I was 8 and my sis was born). I think I was told the names of the genitals and then told that a sperm from Dad goes inside mum and grows into a baby and then it comes out the same way. So I wasn't told the exact details but just a overview. Would a simplified version like that be appropriate?