If that does not work you could try dettol mould and mildew remover (sainsburys green spray bottle) turn the duck upside down and spray a load in side the duck then try and bung the hole with something maybe cling film or tin foil and then turn back over leave to sit or a few hours, (shake a few time during the siting time) and then rinse out by unbunging and filling with fresh water squeeze, fresh water squeeze and so on then try and let it soak. I would try other cleaning products first this will def. get rid of the mould but it's not the best product to mix with children, I would guess, but if worse come to worse or you are willing to clean properly after I do not see a problem.