I think if every child did 10 minutes of something every night, they would improve in that topic, no matter what it is.
The discipline of actually getting them to do it is the hardest. By the time they have argued and moaned and wasted an hour, they could have done whatever they should and more.
The key is routine, but the other issue that we read time and time on family forums, is that we shouldnt be getting our kids to do extra learning at home, that they do enough in school
We have just started with whizzmaths and their biggest emphasis is that a child should do a minimum of 90 minutes a week to improve their maths age. Now for kids that dont want to do stuff at home this is a long time (broken down over 3-4 days ideally, but if you did any kind of maths programme for 90 minutes a week on top of what the kids learn at school, then every single child will improve. They dont need expensive programmes to do this.
However, by paying a third party there is the discipline that work is being set, and expected to be completed, whereas at home, it gets forgotten, delayed and then not done.
I dont know anything about Kumon, but we have used a variety of computer based teaching programmes over th years, starting with the Jump Ahead series.
With learning maths, kids need to understand what the numbers actually mean, rote learning is fine to a point but they need to know what 3x6 or 7x4 or 15/3 really means.
I have read that kids are "great at maths" but cant understand questions when written out as words, eg "there are 24 biscuits and 6 children, how many biscuits each will they get".
If they cant work this out then they dont understand, but if you said was 24/6 they could probably answer, because they have memorised the numbers