What movie is when a kid turns into an cartoon animal?

You're likely thinking of "The Pagemaster" (1994). In this movie, a young boy named Richard Tyler, who is afraid of reading, enters a magical world within a book and transforms into an adventurous cartoon character.

While there are other movies that feature kids turning into animals, "The Pagemaster" is the most iconic one that fits the description you provided.

Let me know if you have any more details about the movie you're thinking of!