In my village library, they had a set of books which were about your body. The set had 3 books, one was for 9 to 11 year old another one for 12 to 14 and another for 15 to 17 year olds. so it approached topics from change of body and body hygiene, to relationship and sex in the books for older teens.
another book was different and it was only about human reproduction , it was based in a comic book story in which two siblings 5 and 7, learn that their mum is pregnant and get explained what happened and what will happen and follow in time till after the baby is born.
it is by Grethe Fagerström und Gunilla Hansson a swedish team so I imagine it is also available in english.
the aforementioned one is obv German but maybe you find a similar english one
https://www.amazon.de/Mann-Frau-Bdn...=sr_1_1?ie=UTF8&s=books&qid=1278409877&sr=1-1
https://www.amazon.de/Mann-Frau-Bdn...=sr_1_1?ie=UTF8&s=books&qid=1278409696&sr=1-1
https://www.amazon.de/Mann-Frau-Bdn...=sr_1_3?ie=UTF8&s=books&qid=1278409877&sr=1-3
so thats the look of the kids , the early and the later teens book.