A previous writeup (whose author has told me they're asking for its deletion) nominated a children's book on sexual abuse named "I Can't Talk About It" and its placement in the children's section rather than in some section for psychologists; as someone who was sexually abused as a child, I must say that the most healing experiences I've ever had have been merely finding out that I wasn't the only one it happened to. (This was in the fantasy novel The Door Into Shadow when I was 14 years old; 13 years later I'm still using the name of the character who was my first hint at not being alone (Segnbora)). Kids who are too intimidated by their abusers to tell anyone what happened need books that are in the children's section, not in psychologists' offices.

Anyway, my vote for the worst children's book ever is "Elsie Dinsmore," an 1868 chapter book rather than a picture book as most of the other votes have been, because it is didactic and boring. It's the first in a series but I've only read the one (courtesy of my late grandmother). Elsie is an annoying goody two shoes and her father is a jerk and I have no sympathy for either of them.