Your child’s body is starting to change, or is about to, and the questions come with it. Pick the one that fits your child's age and what you're ready for them to read. The one thing to know before you start is that these books differ enormously in what they cover. Four of them stay strictly on the body, hygiene, and feelings and leave sex alone; two of them explain sex and reproduction in full. Every book's note says plainly which is which, so you choose with your eyes open.
Pick by your child and by where you want the line drawn. If you want the body facts and nothing about sex, The Care and Keeping of You (written to girls) and Guy Stuff (written to boys) are the classics, staying on periods, hygiene, and the changing body on purpose. Celebrate Your Body and Growing Up Great are the newer, warmer versions of the same, girls and boys in turn, and they also leave sex alone, which is why many parents start with these four. The two Robie Harris books are the ones that explain reproduction in full: It's So Amazing is the gentler, younger one, roughly 7 to 10, and It's Perfectly Normal is the complete, frank one for older children, covering sex, orientation, gender, birth control, and more, illustrated with cartoon nude bodies throughout. Both are widely recommended and among the most-challenged books in America, for the same reason, they hold nothing back. Read each book's own note and decide what fits your child.