I've had my kids this summer write brief paragraphs daily during the week. That has helped their grammar, spelling, and handwriting alot. I think if you just get them writing more and do some spot teaching on areas that need help, that is useful.
I speak German fluently, having studied it for 5 years and then studying at a German university for a year before graduate school. I did not understand the nuances of English grammar until I really studied a foreign language.