This link from Louise Staples has a lot of useful information, but I disagree with the comment on the linked website: 'too much preparation puts children under pressure and may be counter-productive'.
I have no idea how preparation can be counter-productive as my experience indicates that the more preparation the child does, the more likely they are to pass.
It is like saying that too much driving practice before your driving test may be counter-productive, which is clearly not the case.
I have found that children who have done more practice are happier, more relaxed, more confident at school, and more likely to pass. But there must be breaks, of course, and physical exercise.