Great question, Pipsquiggle
I would say it’s important to know as much as you can about the type of test your DC will be taking. It sounds really obvious to say that, but I know people that were trying to teach their children more advanced algebra when we knew the maths questions wouldn’t require anything more complicated than the primary curriculum. Similarly, there’s no point in practising essay questions for a multiple-choice test.
Also, exam technique and experience paid dividends for our DD and others. She came out a bit shellshocked from a mock test, but it focussed her mind on the timings and meant she wasn’t fazed by the setup in the room, the positioning of papers on her desk etc. A friend (who had paid for twice weekly 1:1 sessions with a national tutoring company for a year) felt it was the experience of the mock test that got her DS over the line in the actual exam.
What I wish I’d known a year ago? That, with a little bit of sensible prep, the results won’t be a surprise. In my small sample size of classmates and friends’ children, the children generally considered the brightest passed easily, the borderline children got borderline scores, and the average children missed out. This, of course, is what is supposed to happen (it’s CEM in this area, so theoretically less tutorable) but I found it reassuring that it actually did happen, despite a variable amount of input from parents/tutors. Here, many children sit two tests (both CEM format), one for each LA, and the results were broadly consistent for each child for the two tests.
Best of luck to you and to your DC.