Kumon is expensive and only cover's number topics (at the lower levels anyway), but the repetative aspect of it is hughly helpful in many ways.
My brother did this for a while, he started not knowing 2 plus 2 (at age 11) and finished knowing all his tables, and being able to add and subtract in columns consitently using hundreds, tens and units. He's autistic and had global developmental delay.
My daughter did it for a while and is way ahead of her peers in terms of mental arithmetic. My mum teachs maths and after retiring she ran a kumon centre for a while, which I helped with and be both saw some amazing progress but.......
It's not cheap, the worksheets can be boring, and it is very different to school maths. If you are trying to do it yourself, the prinicple is that you don't move on to the next level until you have mastered the current one.
This is fantastic for children who need overlearing, in terms of moving them on, but I guess can be boring and the biggest problem as a parent is finding the time to make sure its done each day.
The idea is you get a booklet to work through each day, the parent marks it asap and then corrections are done. You tend to go to the class once or twice a week but for me the key was the immediate feedback and corrections and the repetativness of it.