Decay chains are made up of pairs of parent/daughter radionuclide, where the daughter is said to be a decay product of the parent. The branching ratio of the decay pair determines the fraction of the parent which is transformed into daughter.
Ecolego will automatically handle decay (loss of amount/activity due to decay) and ingrowth (the increase of amount/activity in a daughter due to the decay of its parent).
The radionuclide database window allows you generate decay chains by simply selecting the top parent radionuclide. The decay chain can then be edited, typically to remove short-lived daugther nuclides, and then imported to the model. See Importing decay chains