I have PM'd you the criteria used by one of the top grammar school in the country for Creative Writing scoring which you might find useful. I suggest you have a check list of 15-20 parameters and put 3 columns with headings -
Then put a tick mark against all of them to check each piece of writing. The parameters could be - interesting plot, descriptions of settings/characters, correct punctuation, spellings, interesting lines, neat writing etc.
I guess that's the first part. The other part is to guide what the DC could have done in order to get full marks (which unfortunately is where most parent/tutors may find hard).