Hi, since no-one else has replied, I will try with a few simple tips. I would start by focusing very strongly on getting the math basics right, i.e. make sure that she knows all her times tables backwards and forwards and very fast. I would get hold of some past exam papers (on the school website) - but don't allow her to do them yet. Instead, sit down and work out for yourself what she needs to be able to do and what she can do, then work with her to make up any deficit. Once you are fairly sure that has has more or less mastered all of the content, you could start with some practise papers. I would aim to be at that stage by the October Half Term before the january exams at the latest, ideally by the start of the autumn term. In the exam, the papers will, of course, be timed but unless your daughter is very confident, I would start out without timing initially, perhaps even go through the first paper together.
You can get other past exam papers from other schools' websites (try Highgate, Forest etc). There are also mock exam papers available to buy but I don't remember their names - Bond perhaps?
It sounds as if you need to focus on the math but I would look at the English papers also. A big part of passing these exam is having practised taking them, therefore I would also go through some of the English papers and get her to do a few practise papers. These will be much more difficult for you to mark, though, since the answers are not as well defined as they are in Math.
Good luck! I have a daughter at Channing, so feel free to ask more questions or PM me me, either now or nearer the time.