Genius idea! I always take a stab at sizing online (I think men's foot sizes are more flexible than women's), but I still don't know if I'm a 10, a 10.5, or an 11!
I wish my parents had doen this. I definitely fell into the mindset where "oh, I'm smart, so I don't need to try". Needless to say, I'm now playing catch up.
Another consequence of this upbringing is that learning became a grind for me - pursuing intelligence rather than interest.
I burnt out at the age of 12, and the past 8 years have been trying to find direction and motivation for my academic life.
However I do agree with you - I learned math not only by doing it by hand, but also by brute forcing it into my brain. I memorized my times tables from third through fifth grade until I knew them up to 25. I did the quadratic formula by hand until I knew the first five Pythagorean Triples.
These aren't necessarily the "right" ways to learn math - but it certainly made me more curious to find a deeper meaning of why mathematics works the way it does.