Don't worry, you'll be fine. As long as you practise, you're in. None of the kids are actually that bright. They're just not dumb.
To be cynical, all the test does, is screen out bad apples. So, it exposes the kids with dyslexia and dyspraxia, and they get filtered out.
The kids that are left, are the kids that listen well, do what they're told, and take their work seriously.
Of the kids that I know who took the test, all of them made it, except one kid that had ADHD. None of them were exceptionally bright. Some of them I wouldn't have taken, if I ran a school.
So the only thing that matters is, are you trained or untrained! Nothing else. And if you are untrained, it probably comes down to being untrainable or not taking the test seriously, both of which are mortal sins from the perspective of schools that care about league table rankings.
One trick to get your kid to love test taking, is positive reinforcement. However badly they do, congratulate them for going through it, and reward them for it. Make them have an appetite for it. Don't focus on the mistakes, focus on rewarding them for the process. Then they can't wait for the next time.