Thanks for response, this is for 10+ exam, creative writing (Enlgish), essay topic given in exam. For an independent 10+ exam.
I would suggest that DC spends 10 minutes planning it with a mindmap / spider diagram or whatever you want to call it (separate sheet of paper obviously).
In the remaining 50 minutes 2 A4 sheets (or circa 500 words) would be a target.
However I would suggest that quality is more important than quantity (any school setting a 10-year-old a one hour essay is clearly competitive), so emphasis on>
1. Layout - Title, paragraphs, introduction/conclusion
2. Use of adjectives with nouns (no nice or big or good - DC should learn a few favourites).
3. A few adverbs
4. try to work in a simile or two.
5. puntuation, spelling and grammar to recheck
My DS2 is 10, and benefits greatly from re-reading what he has written to fill in the omissions, punctuation etc.
Best of Luck.