Hi nilananthini, do check out some of the sites here: https://prowritingaid.com/art/111/10-Websites-to-Help-Improve-Your-Grammar.aspx I believe that they can help improve your child’s English skills in general which will lead to an improvement in their composition writing.

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!