Here's a book many parents may find interesting and helpful: Scattered Minds by Dr. Gabor Maté
Scattered, written from the inside by a doctor who himself has Attention Deficit Disorder, offers a completely new perspective on ADD and a new approach to helping children and adults living with the problems Attention Deficit Disorder presents.
Several chapters are available at no charge to peruse on the above website.
Attention issues, not necessarily fully blown ADHD, can manifest with children who have written output struggles. It's not surprising that it may be tough for children to maintain concentration when struggling to get ideas onto the page, as fast as they are formulating in the mind and at the same speed as the child's peers who do not have this challenge. Therefore it is useful to read up on attention issues as a means to helping your child and understanding how you can facilitate them to have a more productive and joyful educational experience.