What you have is a form of non-statutory review.
1. Am I entitled to know this year if it is happening for my child?
I would have thought so, but hesitate to say at what point in time. (If there's anything in your child's school record about it, then you could certainly invoke the Data Protection Act.)
2. Is having the child's year 5 exercise books essential (we have them all at home)?
This depends entirely on how the review has been designed. Because it's a non-statutory process, KCC can make up whatever rules it wants.
(In Bucks, they refuse to look at exercise books at review.)
3. If the school does it, and the answer is still a fail, does this make a real appeal by me later in the year harder?
In some admission authorities, if you've had an unsuccessful review, they can argue that you've already been through a process that was 'fair, consistent & objective', and - if the panel agree there is evidence to substantiate this - then the appeal does not go on to consider any other issues.
Fortunately, unless there's been a change, the 'fair, consistent & objective' hurdle is not
used in Kent.
It's not impossible that an individual panel member could note you've already been through an unsuccessful review, and give a bit of weight to it, but I have reservations about this approach because the Appeals Code clearly sets out the procedure to be followed where 'fair, consistent & objective' is not
"3.13 An appeal panel may be asked to consider an appeal where the appellant believes that the child did not perform at their best on the day of the entrance test. In such cases:
a) where a local review process has not been applied, the panel must only uphold the appeal if it is satisfied:
i) that there is evidence to demonstrate that the child is of the required academic standards, for example, school reports giving Year 5/Year 6 SAT results or a letter of support from their current or previous school clearly indicating why the child is considered to be of grammar school
ii) where applicable, that the appellant’s arguments outweigh the admission authority’s case that admission of additional children would cause prejudice."
In so far as one can generalise, my feeling is that most appeal panels would
take a completely fresh look at your case, and judge it strictly on the basis of the arguments and evidence you put forward.