I just got an email from Knowonder since I've dealt with them in the past. They published one of my children's stories online. Now they're upgrading to publishing children's chapter books. Word counts between 10 to 15K. Illustrations are nice and they pay competitive royalty rates. I thought for those who write for that age group, you might be interested in knowing.
Here's more info on it: https://knowonder.submittable.com/submit/20360