The best thing you can do to help your children be better writers is to read with them more! Students who read the most recreationally are generally the best writers. They absorb quality language through their reading, and then it flows out more effortlessly when they're given opportunity to write.
Rather than using endless worksheets, making writing a drudgery, try to create and utilize meaningful, real life writing opportunities. Some of these include journaling, notes, cards, or letters to friends and relatives, and even list-making for the beginning writers.
"The way to write good English is to read it and hear it... In a child, the selection of the better from the worse is not conscious; he is the servant of his word-experience"(5:304) Our speech and writing abilities are highly reflective of the literature and speech we're exposed to. In other words... everything we read and hear becomes a part of our word bank. So, think about what all your children are exposed to. Though some people are more naturally gifted in writing then others, "the inborn gift of style can be starved or stimulated. No innate genius can invent fine language. The stuff of which good style is made must be given to the mind from without and given skillfully. A child of the muses cannot write fine English unless fine English has been its nourishment."(5:292) Be wary of "juvenile literature" which "belittles the language under pretense of being simply phrased for children; as if a child's book like, Treasure Island or Robinson Crusoe or the Jungle Book, be in good style."(5:293) (See "Choice of Reading Material")
- English Grammer Revolution - Step-by-step grammar and sentence diagramming lessons, quizzes and games. Rave reviews.
- Guide to grammar & writing... Another site to check out with grammer and writing lessons
- Grammar book... English usage videos & a few quizzes
- Scott Foresman Reading - He offers free, downloadable grammer & handwriting workbooks for grades 1-6.