Thomas' Snowsuit, by Munsch, was challenged in Lloydminister, Alberta/Sask. because the teacher felt that the story undermined the authority of all school principals. The principal agreed and removed the book from the school library. Another school district followed suit. The cross-dressing of child and principal has raised objections.
Munsch had a librarian call to report that three of his books had received complaints in her school district and she was considering removing them. What she believed was an "organized movement," turned out to be two families in two different schools who had complained about the same books.
Authors also face pre-publishing censorship. Annick Press wanted Munsch to soften up The Paper Bag Princess. Elizabeth, the princess punches Prince Ronald in the nose. Munsch did not give in to pressure from Annick who believed there would be problems with censors. Following its publication, some parents complained that Munsch's The Paper Bag Princess was anti-family because the princess refuses to marry the prince. Other adults have not liked that Elizabeth calls Ronald a bum.
An article entitled Good libraries don't: The Censorship of Canadian Picture Books, tells of a number of challenges to picture books including those of Robert Munsch. In 1988, Munsch reported that, "I have to go pee!"—the repeated line in his book I Have to Go was replaced with, "I have to go to the bathroom."
Munsch said in a 1988 interview:
Kids love talking about peeing and farting, but they do not like stories where the mother says, "Go to hell!" to the father. That's a major taboo violation, and kids do not want an adult doing that. But to have an adult saying "bum" or "pee" that's great. "Underwear" is good too!
Munsch's Giant; or Waiting for the Thursday Boat was challenged in Middlesex and Welland Counties, Ontario for depicting violence towards God. Giant was returned to Middlesex school libraries with the restriction that, "because of the book's religious implications, it won't be in the primary curriculum and teachers won't be permitted to read it to children." Munsch stated that he thought people might be upset over the book on religious grounds but that their letters indicated that they were more upset over the violence in the book. According to a University of Saskatchewan Challenged List, objections to Giant were made on the basis of religion; morality; God portrayed as a little girl; angry Giant says he wants to kill God. Sources: (Roberts, 1996); (CBC4Kids, 2001).
Good Families Don't was the next book Munsch was working on when Giant was being challenged. Good families don't what? Fart. "God knows what the reaction to that is going to be!" thought Munsch. I am unable to find documentation on challenges, so let me know if you have read them. I don't doubt they are out there.
According to a University of Saskatchewan Challenged List, Murmel, Murmel, Murmel faced a challenge. The objection cited is "Human reproduction." Source: (Roberts, 1996)
I am unable to find documentation on Love you Forever but it seems that people either find the idea of the son rocking his elderly mother on his lap touching or weird and creepy. One review on Amazon from an ordinary reader says this,
I was horrified when I read I Love You Forever - another children's book by Mr. Munsch that results in a mother using a ladder to break into the house of her adult son to rock him in his sleep. Because my daughter liked the book, I taped all the wierd [sic] pages together to save the better parts of the story.
Please let me know if you know of a challenge of this book. My feeling is that adults are reading far too much into this touching story. This parent "save[d] the better parts of the story" or so she would like to think.

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