From an interview with David Polonsky, the illustrator:

In illustrating those more sensitive pages, Polonsky says he took great care to make sure nothing was too explicit for young readers.

Frank's musings about vaginas are illustrated with a black and white swirl, inspired by painter Georgia O'Keefe's famously vaginal flowers, he said. When Frank ponders female nudes, Polonsky draws her walking through a garden surrounded by Greek sculptures.

"I took inspiration from Anne's own world. She was really into Greek art and sculptures," he said. "I decided to show the nudes through classical marble sculptures, which I believed six years ago would be a mainstream thing that wouldn't be too controversial."

The illustrations from the book.

Georgia O'Keefe's famously vaginal flowers

This book was out for six years before it became a "problem". Real "someone think of the poor children" bullshit moral panic that is thinly disguising its fascism.