This is the book my parents used with me https://www.amazon.co.uk/Body-Book-Non- ... 0590556088
It's pretty old-fashioned now, but it explains a lot about how the human body works, not just reproduction, which is presented as just part of the story, which I think makes it less of a big deal.
I was 5 when my brother was born and although I don't remember the conversation, I knew how he had come into the world by the time he arrived. My parents just told me the facts, without making me feel that any of it was rude or naughty.
As for 'letting kids find out for themselves', I hope that was a joke. Unless you want confused young teenagers exposed to all kinds of risks they are not mature or informed enough to handle, or picking up playground rumours and unsuitable content from the internet. Ignorance is not bliss.