I know a lot of people are asking me where to send their work, but I can't give a generalized answer to a very broad audience. Even if everyone who read my blog was submitting picture books, it's there isn't one answer to give. My biggest piece of advice is to do research. I may not know exactly where you should submit, but I can give ideas where to start researching.
Identify Your Needs
First you should identify what you are planning on researching. I'm planning on searching for an agent rather than an editor/art director when I'm working to get a book published. My last blog post: https://daniduckart.blogspot.com/2018/10/who-should-receive-your-work.html talks about agents and other general people who might receive your work. I'd like to find someone who likes humor, fantasy, superhero, LGBTQIA+ and animal books. I will also be sending my portfolio work/mailers to art directors/editors so I can possibly get illustration jobs as well. For both of these I want to see what kinds of books they look for, research as much as I can about the company/individual and try to look for someone who has a need I can fill.
There is a lot of information online. For agents: find interviews about them and read their bio. Social media as is important as well. You can tell a lot about a person from their social media! For publishing companies (and agents) look to see what they've done in the past and see how your work fits into what they already do.
Where to Find Markets
Children's Writer's and Illustrator's Market -- Give great information on a number of publishers. Is big but doesn't cover everything. The Artist's Market and Writer's market have some of the same Children's Markets, but has additional market info as well. This is one of my favorite market books. Mine always come with digital access as well, but I haven't used it that much! http://www.writersmarket.com/
The Book Essential Guide to Publishing for Children The digital guide is free and the printed copy is inexpensive (at least in the US) to have shipped to you. This book is much like the Children's Writer's and Illustrator's market, but has huge chapters on marketing your book, and what you need to do before you send out anything. It's 317 pages, so not at all a small guide. https://www.scbwi.org/
#MSWL on Twitter #MSWL is not for submitting anything, but for you to find out what editors, publishers, agents etc. are looking for in a book. Submission goes by their individual guidelines. This is very much more weighted to the adult/writing market. Even then there are lots of agents looking for kidlit and a few looking for illustrators. With the website you can skip Twitter altogether and search directly on their sites for agents you want. http://www.manuscriptwishlist.com/ Be aware that some agents don't certain hashtags so it helps to search similar terms (some will use PB, #PB or Picture Book) it may take several tries to find all the markets for your book. I've also found the unofficial website to be useful: http://mswishlist.com/
#PBpitch on Twitter This is where you pitch your work on Twitter using #pbpitch in the text. These happen I believe quarterly (check their website for more info). They also have a huge list of Literary Agents on their website! http://www.pbpitch.com/ You do need Twitter to participate.
Kidlit411 Tons of information for everything kidlit all on one site! I use this website
#kidlitart on Twitter I was not able to pay attention to much of the market chat on #kidlitart last Thursday. If you missed it as well you can read the recap here: https://wakelet.com/wake/0aea40d6-ae48-4b8e-9973-f339c5fa5326 To particpate in the weekly chats just log into Twitter on Thursdays at 6pm PST/ 9pm EST and search for #kidlitart. Remember to use #kidlitart in your tweets!
Query Tracker This is only towards the bottom of my list because I haven't used it yet! I've heard a lot of my friends using this and they've raved about this. This helps you find publishers and agents for your book. https://querytracker.net/
I know I missed a lot of information, but maybe it's because there are things I don't know! Let me know in the comments what I've missed.