I went to our local Kumon and talked to the owner. She told me that a lot of kids go to Kumon specifically because they work above grade level. She was very specific about the idea that Kumon's approach is that students go at their own pace and are never held back because they're too young.
Kids go for a free placement test (reading or maths), and they spend one or two sessions a week at the center. They have homework, too.
They also have contests and I think they give away prizes (there was a glass case full of what appeared to be prizes).
Val