Hi! I’m a specialist English tutor with several years of experience providing English tuition. One of the best enrichment books I have come across for improving writing skills is this series that is available in Popular: I’m the Best in English Composition Writing. I would highly recommend this series. However, for this series of books to be effective, it would most likely benefit your child to have a couple of sessions to learn composition planning skills, so that he/she will be able to read the model compositions in the enrichment book and understand how the magic of good writing works.
Hope this helps!