Grading rubric for middle school students

We did the Hour of Code this year for the first time and from it, I have elementary classes going through CSF course and I am not grading them. For middle school, we are incorporating coding once a month during their math class. They really liked the Minecraft activity, so we are going to go through all 4. I am going to start by using pair programming - just to make sure they have the foundations set, then each month do a different one. The math teacher wants me to come up with a way we could give them a grade or evaluate them. Does anyone have any ideas?