ground-breaking tv documentary followed three children in medical trials .
they were the first in world to have pioneering treatment immunotherapy .
it uses powerful drugs to kill off cancer cells and destroy bone marrow .
toxicity of immunotherapy means children often need to be sedated .
in the film , the children 's parents reveal why they decided to take part .
the documentary , raining in my heart , will be shown on itv on april 15 .
raining in my heart will be shown on itv on april 15 .