Apologies if this has been discussed before, I've not come across the answer in my browsing - in the exam, where do the children write their workings out? Question booklet, answer sheet edges, scrap paper?

Not in your area, but if you look at the 11+ page on the website of one of the grammar schools, you will find a link to the grammar school's info page, with a further link to the familiarisation booklet, where one of the points of information states:
Candidates can use the question booklet for rough working, but should not write any working out on the answer sheet.

I followed the link from the Marling website, but you can go directly to

https://www.gloucestershiregrammars.co.uk/index.html.

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

Thank you Toadmum, for some reason I'd missed that in the familiarisation booklet! Better hope the boy reads his instructions better than I do!