A large epidemiological study of childhood cancer and power lines in California has been underway for the last few years. It published its results for distance to the power lines in 2016 and has just published results for calculated magnetic fields.
It finds a modest elevated risk for calculated fields: relative risk 1.52. That is consistent with their findings for distance. They conclude:
Our large, statewide, record-based case–control study of childhood leukemia in California does not in itself provide clear evidence of risk associated with greater exposure to magnetic fields from power lines, but could be viewed as consistent with previous findings of increased risk.