The thing about Kumon is that each child is individually assessed and started at a level they can manage. This builds confidence. then the level of work is gradually increased, covering all the basics. If the child struggles, they go back a level and gradually work up again. They have regular monitoring and assessments. The way maths is taught in primary schools is illogical and haphazard IMO, and many children get confused.
My friend is a kumon instructor. She has a degree and a teaching qualification. To run a class of around 30 children takes about 35 hours a week, checking, setting and doing home visits and assessments. She has to go on regular weekend training courses and conferences - (compulsory, or her class will be given to another instructor). She has to cover the entire syllabus and pass all the tests, up to A level standard. She has to buy all the work sheets (which are copyrighted), pay helpers to run the classes and mark work, pay rent for the church hall, and pay her own travel expenses to do all the home visits.
After all that, Kumon take around 40% royalties out of her gross income.
She does it though because it works, and she has a real committment to the children. Many of her class have special educational needs, and have made fantastic progress with Kumon.