My oldest new about birth from about 3 .., then at aged 4 suddenly asked me how the baby got in there I was struggling to unlock our front door at the time and without Much thought just blurted it out He didn't seem to suffer any trauma from the revelation I think we have touched on the idea of seeds coming out of daddy's willy with our twins too, they are just 4 We do have a few books but they're not very interested Just answering questions honestly as they arise seems to be enough I find