There is a difference between "art" and "art education", and that is what
allows one the opportunity (if that is the correct word) to grade (if one
feels it is necessary or if such is required).
I don't like the idea of grading production efforts of children-
especially young children, but I do enjoy the idea of teaching them about
ideas and concepts and challenging them to think and apply what they
know....but outside the context of the product. Of course, mastery of
media techniques can be evaluated and ought to be as long as the
expectations are right for the kids in a given class. We simply have to
be wise and very sensitive to the needs of our kids, the support they
require, and to the outcomes we desire.
As we move up through grade level, more emphasis can be placed on the
more traditional "grading" models that we find in place from middle
school through undergraduate levels.
If the teacher is clear in making expectations clear, teaches the
necessary content, and involves the children with and in it, then
evaluation or grading should not be a problem. The standard set would be
grade-level appropriate, of course, and would be in keeping with those
standards in place within the school.