I created a Kindergarten rubric because all of the primary ones I found did not really apply to the challenges of teaching writing to Kindergartners. My goal for students who come to K with letter knowledge in place is to reach proficient or advanced in all areas: phonetic knowledge, conventions, and idea. For students who start out with no (or very limited) letter knowledge, I am happy to see them progress into the basic category, especially in relationship to "idea". The scaffold of a basic sentence structure seems to help them to write independently and confidently. These samples show the independent writing of one of last year's students. The date the writing was created is written by me in marker.
This one says "pumpkin".
This page says, "I like to count." You can see how much growth was made in just a couple of months. Conventions like spacing and use of lower case letters are not yet present, but he is well on his way to being a writer.
This is the rubric. I am sorry it is hard to read. If you are interested in a pdf copy, you can email me.