It can be tricky trying to teach a subject like grammar to a child too young to really read or write. A lot of people don't even find it necessary, in home schools or public ones. Proper grammar is not only the foundation of writing, however, it's the foundation of speech. In the interest of giving my boys every advantage in life, where their thoughts and ideas will be judged in part by how effectively they are communicated, I started doing grammar lessons with Hunter when he had reached a first grade reading level. I used Jessie Wise Bauer's First Language Lessons for the Well-Trained Mind: Level 1 (Second Edition) (First Language Lessons). The book is broken down into short lessons for the teacher to deliver orally. There is a lot of repetition designed to help children commit grammatical rules and parts of speech to memory; children who remember things easily may find it a little annoying, but it worked really well for our family. It would be easy for any parent or teacher to simply skip over any lessons that are unnecessary once their child has grasped a particular concept.
Here's what the book looks like (click on the image to find out more about it and read the reviews of other parents):
This year we'll be starting the next book, First Language Lessons for the Well-Trained Mind: Level 2 (Second Edition) (First Language Lessons). We haven't done any of the lessons yet (we'll be starting grammar in the fall), but I'm sure it will be very similar and work as well for us.
Once we get the third level, there are also workbooks for the child, who by then should have the skills to start doing some written grammar work.
Another book I've heard very good things about but never used personally is Primary Language Lessons
by Emma Serl. This book is older and covers a lot of classic children's literature. I'm told it works particularly well with the Charlotte Mason method of homeschooling.
If anyone else is using a great grammar book, I'd love to include your recommendations!