SCBWI: the Society for Children’s Books Writers and Illustrators - they have many resources and events. They have a large number of critique groups you can join, webinars, events, and both regional and national conferences. It's a great place to network, learn, get critiques, and find community.
KidLit411: they have a website full of information, but they also have a FB group that is a great place to ask questions.
Underdown/The Purple Crayon: a website run by Harold Underdown, a long-time editor and kid lit industry expert - features a lot of great information.
Storystorm: annual month-long event (FREE) where participants attempt to generate at least one picture book idea per day.
12x12: an online community and webinar series that encourages members to complete one picture book draft per month.
NaNaWriMo: annual month-long event where participants attempt to finish a draft of their novel.
SCBWI: has two national conferences a year in NYC and LA. There are also a lot of regional conferences.
ABLA Big Sur and Cape Cod writing workshops - especially great for anyone looking for an agent or just wanting well-informed critiques.
Highlights workshops: week-long sessions at the highlights property.
Inked Voices: online workshops and paid critiques
Kids Book Revisions: Online workshops and classes run by Harold Underdown
Rutgers University Council on Children’s Literature Annual Conference
Local Writing Groups: For example there is the Writer’s Loft near Boston, and the Writing Barn in Austin.
Workshops and Conferences
Writing Picture Books by Ann Whitford Paul
The Complete Idiot’s Guide to Publishing Children’s Books by Harold Underdown