It is always worth preparing your child for the tests, whether or not they pass.
For some chidren, one-on-one tutoring should be recommended. My daughter is a case in point. I tutored my daughter in math because I have good mathematical ability myself but it required a lot of patience and a perspective of trying to understand the reasons why she was having trouble and then finding a way to help her out of a certain way of thinking. A one-on-one tutor may notice a child's weaknesses or habits whereas a helper in a classroom setting is only telling what to do but not understanding why the child may persist in not doing it (reasons may even be psychological). Subject weaknesses and thinking habits are then worked on as they are uncovered.
When my daughter takes the 11+ test, I've advised her not to let the thought of passing distract her from her task. And her task is to do her best. To do her best, follow the methods she has learned and practiced, don't use anything else (yes, applying is OK but not coming up with something new). Don't mix units, convert, and then convert back if necessary. Mark up your test paper: write down WHAT you are going to do, completely. Then do it. Trust your method and then trust your answer. Don't even look at the possible answers on the answer sheet before you have worked out your answer so that you do not have preconceived notions of what the answer should be. Merely match your answer to the multiple choice answer. At the same time don't assume if you find the answer in the choices, that it is correct. I showed her how I can get almost each of the incorrect answers that were listed as choice for a problem by doing particular things wrong (adding instead of subtracting, giving the amount saved when the question was for the amount paid).
In math, my daughter was in the habit of doing some guesswork especially when under pressure or distracted. Practice is important but the method of getting the answer and being careful along the way are just as important.
For math, she had to learn to get comfortable with decimals, fractions, and negative numbers. I had her practice and memorize 12x12 times tables. I had her practice her 10 bonds (7+3=10 etc) and 20 bonds (14+6=20 etc) so she can doing them quickly. I had to reiterate the definition of even multiplication, adding and subtracting, taking a difference, as well as other basic concepts many many times over. Most importantly, I had HER explain to me HOW she was going about doing a problem so I could discover what was going wrong. Many times we had to go back a step to teaching her some basics that she did not apparently fully understand. It was hard work, but necessary.
I had no idea from the school that my daughter's habits in doing her work were unhelpful in getting the right answer each and every time. She has always been in the top group in her class and all comments about her work have been generally positive. I didn't find out until the summer of year 4 that she needed extra help and only found out when we starting preparing for the 11+ exams (she didn't know her 10x10 times tables at this point). Fortunately I found out then, which gave us a bit over year to prepare. We needed that much time.
Now that she is trusting the math methods by adhering to them no matter what the input, and is more comfortable with numbers other than whole positive numbers, she is way ahead of where she was initially. She has had to work on being careful with her calculations in each step. She has come to understand that getting to the answer may take several steps and there may not be a shortcut. I had to emphasize to her many times over that if you haven't practiced it, don't take any shortcuts, and definitely don't make them up. Understand a shortcut, then practice it, then you can use it in a test because you've then got confidence in it. She has learned not to be afraid of problems that have a lot of information or mixed units by understanding that a problem can be broken down into steps and mixed units must always be converted to the same units before calculations are done with them. We have experimented with measurements. We've taken a measurement from one point to another whilst in the car using the odometer to give her an idea as to how far 1km is. I've had her measure the with of her finger to understand approximatelyhow big a centimeter is. I've had her measure the length between the nuckles of her finger to see how big an inch is. We have gone over and over conversions and how to remember them without memerizing everything. We have measure out 250ml for a cup of tea. We have emptied the juice carton to measure out 1 litre. We have looked at a dice to see what 1 cm cubed looks like. We have tried to achieve an understanding by experimenting rather than doing everything on paper. Some children seem to require this.
I have rewarded her for following a method correctly rather than results. Correct results always follow correct methods, but vice versa is not necessarily true.
If she passes, great; if not, it is not as important as having gone through the preparation and now being set up to succeed anyway. She now knows HOW to learn.