how to grow sweet / not hot radishes
I spoke with the vegetable crop breeder in the OSU Horticulture Department about your enquiry. The pungency of a radish is really a function of the specific cultivar. It's possible to manage the level of intensity by avoiding stress to the radish, which could be induced by water stress, nutrient deficiency, excess heat or damage by mechanical means or insects. So, to get the mildest flavor, grow them under cool conditions (spring crop), grow them in well-drained, friable soil, keep them watered well and avoid any physical damage when cultivating. Keep a sharp eye out for any evidence of insect feeding as well. Starting them early in spring avoids many summer insect pests. Daikon radish is a milder-flavored form that is an option for you as well.