Just for guidance, would like to understand if this kind of score stands a chance of getting into Wilsons?
Firstly, I don't like the term "superselective". It is misleading, these schools are normal schools and the kids applying to them are normal. Do not be put off applying.
Secondly, do not be too judgmental about the results of ANY test the whole point is to get sufficient practice that the candidate is able to perform their best on the day.
Thirdly, it is possible to improve significantly from mock days to the real thing. Determine areas of weakness and do some practice. If coming from the state sector you may well find that your child has not even covered some of the material they will be tested on (entrance tests are before end of Y6).
Finally, there will be some genius candidates and it looks like your child is not in that category but the lowest scoring child to get a place will be far more normal.
You can only do your best, be well prepared and have a back-up.