Hi,
I'm a primary school teacher and taught in Japan for 5 years where Kumon is very popular. I also went on some great teacher courses realted to young children's development in maths.
My advice to parents on Kumon was always, if you want to see your child produce rows and rows of neat calculations, fine, but rote learning does not develop a deep understanding or number sense. At a young age (ie before about 7), it is more essential that children gain understanding of maths in a practical setting. There is actually some convincing reearch that suggests that teaching children standard algorithms before they are ready, undermines their understanding of place value. In my 8 years of teaching, I have seen many cases of this and have called parents in to ask them not to teach them the algorithms as it stops the child having to think for themselves. I've got some articles and stuff on this somewhere (on childcare leave at the moment with my own babies), let me know if you're interested.
Good luck,
Cath