I would start by searching for educational publishers. You can google them or look through the Children's Writers and Illustrators Market. Educational Publishers have their own little icon in the book (an apple, I think, but my copy isn't handy). There are a lot of smaller companies that provide ancillary products (workbooks, activity books, reproducibles) and you might try proposing something there.
I write for the elementary school crowd, so I'm afraid most of my advice runs along those lines.
There are educational magazines geared at teachers that may provide freelance ops if you are interested in writing for the elementary school crowd. Some to check out would be Creative Classroom, The Mailbox (does multiple mags), and Instructor. Other possibilities off the top of my head: Weekly Reader, National Geographic, and Time all produce classroom "reads" for kids. They generally include nonfiction articles and related activities.
Wish I could be more help. I recently learned that the company I write for is looking to sell my publication. <sigh> That will make the third owner in four years and no guarantee that my contract will be renewed for next year. It's a tough market out there now.
Best of luck,
Barb