I know I’m a new member, but lots of people recommend joining the Society for Children’s Books Writers & Illustrators. They have an incredible set of SCBWI-specific resources. They have a huge number of members who also contribute information. There are two national conferences and (probably) hundreds of regional ones. You can get to know

Length of Different Kinds of Children’s Books

How long should book be? Sambuchino, Chuck. “Word Count for Novels and Children’s Books: The Definitive Post.” Writer’s Digest, F+W Media, 24 October 2016. Accessed 26 May 2019. Sff 100-115,000 (aim for 105,000) YA 55,000- 80,000 (but around 60,000 unless sff) Which messes up the book I am writing right now. If this has