I am a mathematician. My children have not gone to Kumon. They will not go to Kumon. Not ever. If they fall behind and need extra tuition (and I don't have the patience) I'll send them to a maths teacher. If they get ahead and need extra stretching (and I if I couldn't do it myself) I'd send them to a maths teacher.
Kumon delivers a fast, accurate and rigid approach to arithmetic. Children who struggle with maths, and who haven't grasped the basics sometimes benefit from Kumon as they don't have to understand why 8x9 is 6x12, they just remember both are 72. For functional numeracy, Kumon may be the answer for the lowest end of the ability spectrum.
I don't "tutor" my children at home, or make them do worksheets or computer games. I do talk to them all the time about mathematical concepts and I answer their questions. Occasionally I set them a challenge, or we have a game that we all enjoy. (Currently the toast game is in vogue chez Quip: every morning at breakfast, the DCs will choose a number and see if it's possible to cut their toast into that number of squares. Every piece has to be a perfect square and all the squares have to fit together with no gaps, into a square. It's a good game for developing mathematical thinking, but they wouldn't do it if it wasn't funny :)) As a result, they entered school 1 or 2 years ahead in maths, and have continued at this level, and love maths at school.
See if you've got a children's music centre near you. Violin and Cello can be learned by reception age children and does stretch them (and may help with maths in a roundabout way).