Next year you won't be so amused!
I can only tell what I did with my child to prepare. I didn't use a tutor. You need to check what tests your child will be taking - Bexley may be changing its admissions - in this year's booklet on secondary transfers, there is a small note to this effect. Unfortunately, it is impossible to tell what these changes will be. For example, it may not have anything to do with actual tests taken but to do with having to opt in, even if you live in Bexley. Perhaps Bexley admissions team would be able to give you an idea as to when this might be announced. But otherwise, you will need to keep an eye on Bexley Council website and this forum (which is really useful for finding out all sorts of information).
Once you know which tests your child is to take (for example, St Os, English and Maths) you can get hold of some work books. I started working with my son at the beginning of year 5. As he did the Bexley test and St Os we worked on VR, English and Maths. With VR, we used Bond Assessment books and Bond and GL-assessment test papers. We also used the Bond book How to do Verbal Reasoning. For English we used Bond books on Comprehension and English, together with Bond book The secrets of Writing (good for KS3 also!). Finally, for Maths we used Bond Assessment books and Bond and GL assessment papers.
We worked for about half an hour each week, for each subject. Also you must encourage your child to read widely and perhaps do things like suduko (which my son really liked).
I could go on.... But might lose the will to live! Good luck for the future. I have to admit that I feel slightly stressed at the moment (waiting St Os results today). My son has passed Bexley test, but distance to school may or may not be an issue. The main thing though, is by taking the tests, your child may have extra choices about where he or she may go to for their secondary education, which can't be a bad thing.