We bought our daughter a similar book when she was about 7/8 i think, answered all her questions, all fine...

But she used to sit with her younger cousin and read her stories when they played together. Guess what book she decided to read from one day!
I was mortified. Fortunately, my bro-in-law and his wife saw the funny side and weren't upset at their daughter's impromptu sex ed session. I think they were relieved it saved them from having to broach the subject!