I started both of my girls in grammar studies using First Language Lessons, a very simple grammar study that focuses on the basics of English grammar and writing. My daughters memorized poems, learned the months of the year, memorized the definitions of the parts of speech, and explored narration. Even today I can start one of the memorized lists or definitions, and my daughters chime in with me!
At 3rd grade, I started formal grammar studies using Rod & Staff English. I absolutely LOVE this curriculum! It thoroughly covers grammar (including diagramming), AND it incorporates the development of writing skills including letters, book reports, poems, short stories, etc. I love the teacher's manuals that give me ideas on how to introduce the new concepts and provides a quick oral review before each lesson. Because it was created for classroom use, I usually don't have my students do all the exercises unless they are needed. But, I love that there are also worksheets that I can purchase to help when the exercises are not enough.
There are lots of other grammar curriculum out there, but when you find what you like, why change? All the major curriculum providers offer grammar studies - Abeka, Bob Jones, and Alpha Omega. There are also other programs that use non-traditional methods, such as Shurley English or Easy Grammar. Also, you can find many FREE resources online. Click here to view grammar links that I have collected.
Please be aware that many grammar curriculums do not include writing skills. You will have to incorporate writing in a different way or through an additional writing curriculum. Many people enjoy the Institute for Excellence in Writing and Writing Strands. Click here for other writing resources.