All writers and illustrators should join.. the Society of Children’s Books of Writers and Illustrators
A go-to site for writing, illustrating and publishing Children’s Books is Purple Crayon by Harold Underdown.
If into Non fiction picture books then take a look at Kristen Fullton’s site – Explore Nonfiction.
A great Young Adult literature site is – http://www.adventuresinyapublishing.com/.
For all things about diversity in children’s literature, go to – WeNeedDiverseBooks.com
Querytracker.net is a great tool when you start querying.
If you are looking for professional editing, then look no further than editor Emma D Dryden of drydenbooks, bestselling author Emma Walton Hamilton, and author and editor Harold Underdown and author and editor Lorin Oberweger at Free Expressions.