Thinking about percentages can put you off before you even start.
Yes your child must be bright but it all needs to come together on the day.
Lots of practice, speed and accuracy, a positive response under exam pressure and luck on the day all play a part. Not forgetting the basic maths, english and logic (NVR) skills.
It is a 'big ask' but if you don't try you will never know, the journey is as interesting as the end result, we learnt a lot about each other.
I would suggest that you gently start preperation and see how it goes.