I haven't personal experience of Kumon, but know friends whose children go, and also a friend who teaches it. Confidence building is what they claim to be good at. It starts very simple - basically adding ones; so the child gets to have some success at whatever the level they are at.
It is expensive (IMO), but many venues to offers (like free registration fee) in September (which is a heavy recruiting time for them) - so do ask about those.
Many places also do Kumon English too atm, but don't get talked into doing that as well, unless you want to. With the Maths the idea is the child does a worksheet: 10-15 mins EVERY day, two of which will be in class with the Kumon teacher. It takes a lot of motivation and enthusiasm to do this, and to keep it up for any length of time. So you may want to aim to do it, say, for a term; and really focus on keeping the motivation up, but then review it then to see whether you think she is benefitting, and whether you think she will continue to benefit or not.
I think a lot of the success of Kumon depends on the backing and support from the home, and whether the child's personality suits that sort of programme or not.
HTH
Do ask if you want to know any more.