House inside my Mummy might be a bit young - my 2yo likes that one. Mummy Laid an Egg is my 4yo's favourite. He also likes Where Willy Went. We also have a couple of books with no words - My New Baby and another one by the same author. They are quite good for talking about what is happening/what will happen rather than just reading the story.
Thanks, I'll have a look at those. It's totally new territory, DD has never asked at all where babies come from despite a clutch of new cousins in the past year. Maybe it being a bit closer to home will inspire her to ask!
Thing is that DS really isn't all that interested in that page at all. He loves all the silly things the parents say first and feels all smug that he knows better. The good thing about the sexual positions page is that it shows sex as fun, which not many books do imo.