What does the book Frindle teach small kids?

Frindle can be seen as teaching children how language develops and how it works. Ultimately, words in themselves mean nothing; meaning is derived from how they are used. We can look up a word in the dictionary and find out its definition, but the way we use words on a daily basis gives them their deepest meaning, one that forms the basis of our communication with each other.

Mrs. Granger has a somewhat pedantic understanding...

Frindle can be seen as teaching children how language develops and how it works. Ultimately, words in themselves mean nothing; meaning is derived from how they are used. We can look up a word in the dictionary and find out its definition, but the way we use words on a daily basis gives them their deepest meaning, one that forms the basis of our communication with each other.

Mrs. Granger has a somewhat pedantic understanding of words, deriving from her education and work as a teacher. That object the children use for writing isn't a "frindle;" it's a pen, because that's what it says in the dictionary. The author isn't suggesting that it's wrong to use the word "pen." After all, its own meaning has also derived from its use over time. But the point that's being made in the story is that the word "pen" arose in the exact same way as "frindle." And eventually we find out that Mrs. Granger understood this all along. She deliberately prohibited the use of "frindle" by her pupils because she knew it would become popular. She knew that widespread use of the word would give it its meaning.