I'm new here, but I'll try to help! Sorry if I break any mumsnet etiquette rules.
I'm a former Kumon instructor. There are pros and cons as discussed previously. It works well if you are completely focussed and strict with it , your kids will rebel, they won't want to do it every day, if u think u can deal with this then that helps.
As a system it works, my kids are now near the top of the programme and are in gifted and talented maths at school BUT it doesn't help their problem solving skills, it's expensive for what it is and the carting of your kids to the centre weekly can be a huge time suck and I loved working with my students but rarely got to spend more than 5-10 mins with them if that.
Whether you're looking at Kumon, Kip, private tutoring, or helping your child yourself none of these things are quick fix solutions, they all take time and as parents we don't feel we have time and want quick results.
My advice, try whichever for 6 months but have a target in mind - ask the tutor, teacher, instructor what you can expect after 6 months and how you can support that goal and then review the results after that period of time.
I've just launched a blog www.mathsinsider.com which looks at ways in which parents themselves can help their kids maths and gives advice based on the questions that hundreds of people have asked me about supporting their kids maths. Have a look, there just might be something useful there for you