Stop agonizing over grading and use Rubrics.
It spells out step by step what is expected.
Then the student chooses what grade they want to earn.
The higher grades require higher order skills.
It also makes the students know that they are in charge.
I use them grades 3-12.
For grades K-2, it sounds like you've got a good system in place already.
List, demonstrate, and display the skills students need to use
and grade the students for following your directions.
The outcome is not as important as the process.
Good luck and good art.
Patti