Complete an assessment of grammar and punctuation before focusing on improving skills. There are numerous grammar assessment tools available online. Use the Funbrain website to help students learn the basics. For younger children start by learning nouns and verbs. The interactive site gives children definitions of nouns and verbs, and provides lots of practice in identifying these parts of speech.
Review the fundamentals. For college-level writing and more well-developed high school writing, the Online Writing Lab from Purdue, OWL, is the website of choice for most teachers and parents. Separate learning sections focus on parts of speech, punctuation, common spelling errors, tone and diction. A child should have a working knowledge of the English language and be able to use written English in order to use the site effectively.
Read online newspapers, specifically papers that offer a "Letters to the editor" section. Give the child an opportunity to find grammatical errors in these letters or articles. Have the child apply what she has learned from her study by re-writing these articles and letters and then correcting them.