Reddit reviews Human Body Theater: A Non-Fiction Revue

I read mine Human Body Theater (Amazon link). It goes over the functions of all the organs in the body. In my experience, knowing that all the organs do something different and unique made it really easy to explain what the uterus, vagina, penis, testicles all do without any issue. It's for older kids, but my kid got basic info out of it at 3 years old, and found it entertaining.

I also got her the book How You Were Born (Amazon link) which has photos of a baby in utero, and shows the various stages of gestation. It glosses over how the baby got in there, but it's a good start.

Lastly, this book The Amazing You! Getting Smart About Your Private Parts (Amazon Link) has diagrams and goes over all the functions of what each set of genitalia does. It's great for your preschool age kids.