The adverb visually means "related to the sense of sight." For a kids' book to be visually interesting, it has to have illustrations to look at, and not just words.
If a person is visually impaired, their ability to see is limited. And if an art exhibit is visually overwhelming, there's just too much for your eyes to take in at once. A critic who describes a movie as "visually stunning" simply means it's very nice to look at. Visually comes from the adjective visual and its Latin root, videre, "to see."