From the Japan govt’s official statistics of children cancers:
The country has been taking statistics on pediatric cancer since 1975. Every year, 2,000 to 2,500 people were affected.
The least was in 2006, where there were 1861 people affected, the year before the Niigata Chuetsu Earthquake.
But the numbers of children cancers really much increased after Fukushima. In 2015, 3246 people, in 2016, 3161 people, in 2017, 3279 people.
I think the incidence of children cancers will continue to increase in the coming future.