Can you say, “don’t worry. You’ll find out when you’re older.”
I’ll second these books – we got them for MilliCal when she was young (except for the “Not the Stork” book – never heard of that one). We had gone to Disney World and went through the “Miracle of Birth” show (or whatever it was called), and the thing about where babies come from started a veritable tempest of questions from little MilliCal. She read up on it with geat enthusiasm.