My very inquisitive 4 year old asked .... and I was caught on the back foot ...... so I said - "How do you think the baby got in Mummy's tummy"

....and he said "Well, when you and Daddy are in bed asleep, cuddling, the baby crawled in there".

I told him that was pretty close - and he was happy. I have found that when my children ask the tough questions, it's pertinent to ask a question back at them.