There are so many options that there really are too many, aren’t there?! One suggestion I have is to do the learning yourself. Library books, online websites, you have resources at your fingertips to learn what an expository essay involves, what a persuasive essay involves, what are the parts of a letter, or different rules for poetry forms. Don’t go crazy over it. Pick one. Figure out what it is, write it yourself – more than once. Then introduce it to your child. Let them play with it and try it out – more than once. Then you start learning about another one, try it more than once, and pass on that experience to them.
For actual curricula – we like Brave Writer (the philosophy and products) and Write Shop Junior (has great games built in which some of my kids enjoy) for anything below high school. In high school we do 1 year focusing on academic writing (essays, etc) with The Power in Your Hands by Sharon Watson. And fun writing – like One Year Adventure Novel. And then whatever fits the child’s interests and goals.