From what I’ve gleaned from attending conferences, it feels like a large percentage of the people writing children’s books are white women in their forties, fifties, and sixties with at least some college education. This probably has been the case for decades. The children’s book authors I’ve met often share a few characteristics: They grew up with books, they’ve read a lot, they have some sort of connection to children, either through teaching, parenting, or a job that puts them in contact with kids, and they’ve had the time and money to develop their writing skills and attend conferences. So I think there is definitely a socio-economic factor in becoming a children’s book author.
For a very good list of reading about diversity and children’s literature see this post at Philip Nel’s blog.