Is Yertle the Turtle and Other Stories the first children's book to use "burp"?
As said by Dr. Seuss himself about that book:
"'I used the word burp, and nobody had ever burped before on the pages of a children's book. It took a decision from the president of the publishing house before my vulgar turtle was permitted to do so.'"
Stefan Kanfer (October 7, 1991). "The Doctor Beloved by All". TIME. ISSN 0040-781X.