IMO, if your DC needs a lot of coaching to get into a school, then maybe it isn't the right school for her/him.
But, as most children trying for the tests seem to have some kind of coaching, if yours are not familiar with the types of tests used, then you put them at a disadvantage.
For DD1, who is now at a superselective grammar we found practise tests on a website called Elevenplusexams
There are some free papers, and many that you can pay to download, as well as paper books you can buy. My DD found some books from an organisation called Learning Together to be the most useful, as they break down the kind of problems that you can expect to come across, so you can work up from the simplest of a "type" up to the most difficult. But we didn't test everything available, of course!
We concentrated mostly on VR and NVR, as these were the tests needed for our first choice school. We did very little on maths, as I thought she was probably doing enough at school (she was in a group working on the Y7 syllabus). But for our second and lower choice schools, she needed maths, and she did not perform anywhere near as well for maths as for the other two. However, it is my belief that this is because her strengths are in other areas, and not that we should have done more "coaching".
I can't remember precisely, but I know she did about one practise test each day for the summer holidays before the tests, and about one or two a week for the term before that, and the part of the autumn term before she sat the tests. She was very motivated to do it, as she really wanted to get into the school.
(Then we went out to dinner to reward her for all her hard work, long before we knew whether it had paid off!)