"My DC and I use textbooks to discuss problems in maths, science and philosophy every night."
In effect your dc is getting a one to one tutorial every night with someone who (I presume) is highly numerate/a mathematician.
This is exactly what I mean when I say that tests at 11 will not identify potential which hasn't already been developed by parents and teachers.
The concept that we can separate off those children who achieve highly at 11 as though they are somehow fundamentally different from all other children.
I know other children who are also being hothoused (by which I mean bought up in a family where there is a constant and fairly intense focus on attainment of knowledge and the development of skills which are academically useful). I'd suspect that a very high proportion of children at grammar schools come from families like this. The children aren't fundamentally different from the other children on the top table in primary except insofar as the amount of educational input they've had at home.