Nobody stopped her from writing the book
Kid is in favor of censorship (an action by government) and bookburning.
In favor of books being in proper places.
Which is the issue here
What grade do you think is good for teaching about slavery?
What grade do you think is good for teaching about the holocaust?
What grade do you think is good for teaching about racism?
What grade do you think is good for teaching about sexism?
And at what ages/grades do you think kids start observing or experiencing any of those?!
In every grade the issues can be presented and discussed at an approriate level considering the developmental level of the children.
As to what grade kids begin experiencing some kind of social bullying, which is the root of the issues you are raising, that would be about the middle of first for many, and by the end of Kindergarten for some. By 2nd grade, kids have decided who they can pick on, who can be ostracized, and with a rationale that their peers will accept. This is why such necessary education is so important to begin at a young age.
However, no curriculum can be as effective as the one in which the community at large has bought into. Therefore, it is incumbent upon school boards, school leadership, school unions and their members, mayors, town councils, etc, to reflect that values that they say the students should display themselves.
And that is so often NOT the case.
We have met the enemy and he is us.