Who cares about that, a kid in yr8 shouldn't be doing anything other than maybe basic quadratics and trig. It's yr8! One of the very last years with pretty much no substantial homework. Go outside and play a sport or something, don't get bogged down over little booklets filled with calculus.
kumon was the reason i did well in maths in year 12
hmmm, interesting conflicting views here, but yeah :\
What's your view out of curiosity? Would you encourage your younger siblings to be able to do volumes of revolution by late primary/early secondary school?
My view is a bit biased, as I did Kumon for about a month in primary school (grade 1) but my parents pulled me out because they too believed "kids should be kids". I distinctly remember them taking me out to a movie instead of going to the next Kumon class (they didn't tell me I was to stop going), saying that to me. Furthermore, my yr8 maths teacher drilled a guy in my class (the same one as aforementioned) for being a "monkey with no tricks" after using the Kumon rote-learning techniques (which I don't think is the right way to learn secondary school maths either).
Getting some hands on experience with complicated stuff at a young age certainly helps. Even if you don't fully understand it or you develop bad habits in the area (e.g. programming for example), I've found that the sheer experience of "seeing it all before" gives you an inherent advantage.
This isn't really about a head-start, this is more about extreme parenting. Whilst it may make things easier, there is more to success than rote-learning how to solve problems. And I don't really think that this is too much of an advantage, I wouldn't trade hours of my childhood for Kumon.
I'm against it, you can only be a kid once, and personally, I'd live those memories to be filled with fun rather than work. There's plenty of time for work later in life imo.