fz
I just asked my dd1. She said can still remember decimals, it's the concept, abit like swimming, once get it won't forget. But somehow multiplication table is different lor, really need to memorise by hard. Don't know why my dd2 has been learning multiplication for years & can still get it wrong. She learned, forgot, re-learned, forgot again. That's the part where I think Kumon repetition is useful. My dd1 kept repeating those worksheets till multiplication was completely drilled into her whereas dd2's abacus didn't require that sort of repetition so somehow her retention is not permanent.
dd2 心算 is quite good, she can calculate 17+8 in her head if we ask her. So abacus is not entirely useless lah. But her accuracy is really bad. Always add wrongly when doing school worksheets even though she should know the answer. I've been 'threatening' to send her to kumon, haha!
But repetition is very tedious lor, might want to discuss with your k2 boy, get his consent before embarking on kumon. My dd2 refused to give her consent.