I always thought it meant different races and cultures eg Asian, Chinese etc and not just white characters written about. But since I've started writing about autistic children, I now know there is more to it. It can mean disabled children in books, not just physically disabled but hidden and invisible ones, too. Eg autistic children might look normal but they are not inside. Also, deafness and heart problems. They all affect people and are not visible, unless someone is wearing hearing aids.
There is another way to help diversity in writing, especially if you are an indie author like me. As well as writing about these different characters, you can choose which font to print in and how much space to have on the page, to make it easier for children with learning difficulties to read. My current children's books (The Rainbow School) has large font because I was told that was best for that age range (7-9s). When I published via Createspace, I found there were big spaces between some pages (where I had written long paragraphs which didn't fit on 1 page). I wasn't happy but then I got thinking. At the retreat in May, I sold a copy of Billy to a new friend, who bought it for her friend's daughter who was older than the target age but had learning difficulties. She liked the book. So, now I feel that even thought it wasn't intentional to have those gaps, it is a good thing to have, because it makes the books more accessible and available for a wider audience than I intended. I shall try to keep this in mind for the further children's books.
What does diversity mean to you with books?