We did Kumon for awhile. My kids found it frustrating because of the endless drill. They had to start at a level or more beneath what they knew (because they not only had to get the answers right, but fast). Then, they had about 200 pages of worksheets that they had to complete, and if these had mistakes or weren't fast enough then they got them again. Each new level was only slightly harder than the last and resulted in another 200 pages of worksheets. We spent about a year doing it and when we finished my sons were just getting to the point where they would soon learn something new.
It did help solidify the times tables for my one son, but most gifted kids hate that drill and kill approach and my youngest was seriously starting to get turned off math.
Our neighbour's son, who is good in math and is in public school, is getting some challenge now beyond what he would get at school, but he's been doing Kumon for more than two years to get to that point.
Do you have a university nearby? Maybe someone there runs a math club for elementary kids.
